Why this answer matters
Older MacBook Air generations had different multi-display limits, so using advice written for another chip generation can send troubleshooting in the wrong direction.
3 requirements4 verification steps2 primary sources
Requirements
What has to be true
- Use the MacBook Air's Thunderbolt ports for the external display connection.
- The cables, adapters, docks and monitors must support the requested resolution and refresh rate.
- Apple documents up to two external displays for M4 MacBook Air; exact resolution/refresh combinations still matter.
Verify it yourself
Use this test sequence
Change one variable at a time so you know which requirement made the difference.
- 1
Identify the Mac model/chip first in About This Mac.
- 2
Test one monitor directly before adding a second monitor or dock.
- 3
If one monitor works and the other does not, swap cables/ports to see whether the failure follows the monitor or connection path.
- 4
Check macOS Displays settings after every meaningful connection change.
Limits
What this answer does not prove
- A dock advertising multiple outputs does not override the Mac's supported display topology.
- Scaled resolutions, HDR and high refresh-rate modes can change which combinations are available.
Relevant DeviceOK checks