CKL CKL-42MVKVM-A

Price: $389.00

Computers: 4 (or 2 dual-output, or 1 quad-output)

Monitors: 2, OUT-A and OUT-B, not interchangeable

Video: HDMI 2.0

Max resolution: 3840x2160@30Hz or 1920x1080@60Hz, fixed

1440p: not supported at any refresh

Both outputs forced to the same resolution

EDID: two fixed profiles, toggled by button

Switching: hotkey, mouse click, wired remote, panel button

USB: 4x USB 3.0 Type-B in, 2x HID + 2x USB 3.0 out

Audio: 2x 3.5mm, de-embedded from HDMI

Size: 200 x 75 x 42mm, 0.65kg, metal

Power: 5V DC, 5W

Warranty: 1 year

The switch, 4x HDMI 2.0 cables, 4x USB 3.0 cables, the wired remote, a power adapter and a manual.

That's a genuinely generous bundle. One owner put the cables alone at 50 dollars or more.

The remote runs about 5.9 feet and CKL says it extends to around 50 feet with a longer 3.5mm cable.

One warning from page one of the manual: power-on order matters. KVM first, then monitors, then computers. Get it wrong and you'll be troubleshooting a blank screen that isn't a fault.

One owner reported receiving no printed manual. Another did.

This is the bit the spec sheet doesn't explain, and it's the reason to buy the thing.

OUT-A is your work screen. Always one machine, full screen. Switching OUT-A also moves the keyboard, mouse, USB 3.0 hub and audio to that machine.

OUT-B is the wall. It cycles through layouts: equal four-way split, one large plus three small, picture-in-picture, dual view, single full screen, or mirror.

So you work full screen on one machine while watching the other three. Same architecture ATEN uses on the CM1284 at nearly six times the price.

With two dual-GPU machines instead of four, you also get extended and splicing modes.

Yes, but not the way you're imagining.

CKL calls it Through-screen Operation Mode. The cursor moves between PC1 and PC4 up, down, left, right or diagonally.

The catch is you don't just drift across a border the way you would on an ATEN. You hold the middle mouse button down, drag to the target pane, then release. Hotkey mode has to be on.

It works. It's also buried behind a gesture nobody discovers on their own. One of CKL's seven reviewers asked for a feature the unit already has, which tells you how findable it is.

There's also a sync mode that drives all four machines with the same keystrokes at once.

HDMI 2.0 carries 18 Gbps. 4K60 at 8-bit needs 17.82 Gbps on the wire. It fits, barely. That's why a plain HDMI 2.0 KVM does 4K60.

The output isn't the problem. A multi-viewer has to receive four streams at once, decode each to pixels, scale them, hold them in memory and composite a new image.

A 4K frame is 24.9MB. Writing four of them plus reading for compositing needs roughly 7 GB/s at 30Hz and 15 GB/s at 60Hz. One cheap DDR3 channel achieves about 4.5 GB/s.

4K30 fits on a narrow memory bus. 4K60 needs a wider one, which means more pins, more layers, more money.

The ATEN CM1284 costs 2,254.95 dollars at B&H. Four in, two out, same architecture, 2-year warranty, true boundless mouse switching.

Its multi-view ceiling is 4096x2160 at 30Hz.

Same 30Hz. Nearly six times the price.

The AV Access 4KMV41 at 199 dollars: 4K30. SIIG's 2-port at around 600: 4K30. Rextron's QSKEM-2014: 4K30.

The only units breaking it are newer silicon with one output instead of two, like Rextron's QSKM-3114 and the OREI UKM-401MV.

So if you want 4K60 multi-view, you give up the second monitor. That's the actual trade, and nobody in the category states it.

Text stays sharp at 30Hz. The pixels are exact. The problem is scaling, not refresh.

A 4K source shrunk into one quadrant of a 4K screen lands in 1920x1080 of screen area. Half linear scale. Nine point text becomes four and a half point. You can see a machine is doing something. You cannot read it.

The fix nobody tells you: set the three watched machines to 1080p. The quadrants become pixel perfect.

Cursor position updates every 33ms instead of 17ms, plus the scaler's own delay. Expect noticeable pointer drag and overshoot on small targets.

Also gone: HDR, VRR, 1440p at any refresh, and per-monitor resolution.

The manual's own FAQ is more useful than any review, because CKL pre-announces its bugs.

Question one: if the keyboard and mouse stop working after a while, power cycle the switch so it re-identifies the USB devices. That's a vendor telling you about a USB enumeration bug up front.

Gaming mice and multi-function keyboards may not work on the HID ports. Move them to the USB 3.0 hub, which also kills hotkeys, mouse-click switching and through-screen mode.

Audio is fragile. You must pick HDMI audio in the OS, and in split modes it defaults to PC1 only. CKL's own workaround is a USB sound dongle.

Blank screen after install is question two. Press the EDID button.

One year warranty. ATEN gives two, SIIG gives two, TESmart gives up to five. CKL is the shortest in this group.

Returns are 30 days with a 20 percent restocking fee, and you pay return shipping. Original shipping is not refunded.

On a 389 dollar switch, changing your mind costs about 78 dollars plus freight. That's the harshest term of any unit in this comparison and you should know it before ordering.

Defect claims need a video of the fault emailed within 30 days.

US returns go to Irving, Texas. International returns go to Shenzhen.

CKL 942HUA-3, 199 dollars: same brand, same four computers and two monitors, but 4K@60Hz, EDID emulation and hotkeys. No multi-view. Winner for most buyers. The 190 dollar premium on the -A buys exactly one thing, seeing all four at once, and costs you 4K60, 1440p, HDR and low latency.

ATEN CM1284, 2,254.95 dollars: true boundless mouse switching, daisy-chains to 16 sources, 2-year warranty. Same 30Hz ceiling. Winner if budget is not the constraint.

OREI UKM-401MV, 219 dollars: claims 4K60 multi-view, seven modes, optical audio out. One output only, so no full-screen work monitor. Winner on price and refresh, loses the whole two-output idea.

Mouse Without Borders, free: cursor and clipboard across four Windows PCs, each on its own native screen. Winner unless you physically have fewer monitors than machines.

389 dollars direct from CKL, in stock. 4.7 stars from seven reviews, hosted on CKL's own site.

Watch the model number. There is a CKL-42MVKVM without the A at 299 dollars. The 90 dollar difference buys keyboard hotkey switching, mouse-click switching and through-screen mode. Search results mix them up constantly.

A Newegg marketplace seller lists the cheaper non-A model at 399.28 shipping from China, which is more than CKL charges direct for the better unit. Buy direct.

Also ignore one bullet on CKL's own page claiming 4K@60Hz. The manual says 4K30 and 1080p60 only, and the manual is right.

What order do I power things on?

KVM first, then monitors, then computers. It's page one of the manual and it prevents a blank screen that looks like a fault.

How do I switch?

Double-tap right Ctrl, then the port number, then Enter. Remappable to Num Lock, Scroll Lock or left Ctrl. Or use the panel button, the wired remote, or click into a pane.

My keyboard and mouse stopped working.

Power cycle the switch. CKL documents this as expected behaviour, not a fault.

I have no picture after installing.

Press the EDID button. It toggles between a 1080p60 and a 4K30 profile.

Can I read the quadrants?

Not if the sources are 4K. A 4K source in a quadrant lands at half linear scale and nine point text becomes four and a half. Set the watched machines to 1080p and it's pixel perfect.

Does the mouse move between panes?

Yes. Hold the middle mouse button, drag to the pane, release. Hotkey mode must be on.

Will 30Hz feel bad?

For supervising machines, no. For working, yes. Expect visible cursor drag.

Can I run 1440p?

No. Only 4K30 and 1080p60 exist on this unit, and both outputs are forced to match.

Do I actually need multi-view?

Only if you need to watch four machines at once. If you just switch between them, CKL's 942HUA-3 at 199 does the same ports at 4K60 for half the money.

What's the difference between 42MVKVM and 42MVKVM-A?

Ninety dollars, and the A adds keyboard hotkey switching, mouse-click switching and through-screen cursor movement. Worth it.

Can I return it?

Thirty days, 20 percent restocking fee, you pay return shipping. About 78 dollars to change your mind.

Is software cheaper?

Mouse Without Borders is free and does four Windows PCs. It cannot show you four machines on one screen.