MacBook M4 dual monitor

Macs will not drive two displays over a plain USB-C dock. If you want to drive two displays from a dock, using hardware-supported video output, it will need to be a Thunderbolt dock. Other World Computing, SonnetTech, and CalDigit all sell such docks.

As for driving two external monitors and the built-in display at the same time, the Technical Specifications for both M4 MacBook Airs indicate that this is possible. Note: M4. The M1, M2, and M3 MacBook Airs cannot do this.

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

MacBook Air (15-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"

I'm not going to recommend buying used. I have no way of knowing how it might turn out – and I don't want to make guarantees.

I have not used the Thunderbolt dock products of any of the three companies that I listed, but I do know that all three have been in the business of selling Thunderbolt gear to Mac users for a long time. Presumably all have a fairly good knowledge of Macs, and the ability to provide good tech support to Mac users.

As for Thunderbolt 3 vs. Thunderbolt 4, you might find this article interesting.

Other World Computing – Blog – What’s the Difference Between Thunderbolt 3 and Thunderbolt 4?

I wasn't aware of some of the finer points raised here until I happened to come across it.