Connecting two external monitors to MacBook Air M4 via Satechi Thunderbolt 4 hub

You're running your 3840x2160 pixel mode in [Retina] "like 2560x1440" mode. In this mode,

• The Displays Settings resolution (a.k.a. UI Looks Like resolution) is 2560 x 1440. Retain-aware applications use this for sizing, and non-Retina-aware legacy-applications actually believe it to BE the display's resolution.
• The Mac draws things on a 5120 x 2880 (5K) canvas.
• The Mac downscales the contents of the 5120 x 2880 pixel canvas to 3840 x 2160 pixels to get the signal that it sends to the monitor. What is going over the wire to your monitor is a 4K signal – so unless there is a bottleneck in the M4 chip or the Mac BEFORE OR AT the point at which the signal gets onto the wire, you should be able to drive two 4K monitors, running in this mode, from a single Thunderbolt dock.

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MacBook Air (13-inch, M4, 2025) - Tech Specs - Apple Support

"Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

• Up to two external displays with up to 6K resolution at 60Hz"

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That said, I haven't tried it. You could experiment by

• Running both monitors at native resolution (3840x2160) and seeing what happens
• Running both monitors in "like 1920x1080" mode (where the internal canvas has 3840x2160 pixel resolution) iamb seeing what happens
• Connecting one monitor to the dock, and the other directly to the MacBook Air, and then running both of them in "like 2560x1440" mode and seeing what happens

drfj wrote:

I am running the MBA closed in clamshell mode, so I am not trying to get three displays going at once, just the two external.

You would need to close the lid on a M3 MacBook Air to get support for two external displays – but you have a M4 MacBook Air with a more capable processor. It can drive two external displays with the lid open.

I should add that if you have a 4K monitor, and are using "icon view" in Displays Settings, it is entirely normal for all of the choices except "More Space" to involve Retina scaling, so as better to make use of your monitor's resolution.

E.g., on a M1 Max Studio with a 4K Dell monitor, the choices are

• 1920x1080 – draws on a 4K canvas
• 2560x1440 – draws on a 5K canvas with the same resolution as a 27" 5K Apple Studio Display
• 3008x1692 – draws on a 6K canvas with the same resolution as a 32" 6K Apple Pro Display XDR
• 3360x1890 – draws on a canvas with more resolution than a 32" 6K Apple Pro Display XDR
• 3840x2160 – draws on a 4K canvas

In all cases, the display (connected via USB-C) reports receiving a 4K signal.