Berk_Y wrote:
Thunderbolt Display works fine when I plug it into my 2015 MacBook Pro without an adapter.
All of the 2015 MacBook Pros have Thunderbolt 2 ports, with old-style Mini DisplayPort connectors, that can act either as Thunderbolt ports or as Mini DisplayPorts, depending on what you plug in. So you would not require an adapter.
Assuming that this is a Thunderbolt Display with a three-headed hydra cable – and not a very similar LED Cinema Display with a two-headed one – the Apple Thunderbolt 3-to-2 adapter is the correct adapter to use.
If you are using the correct adapter, there is one possible reason why things might not work which I have not seen mentioned in this thread. Recent versions of macOS reportedly do not want to see display transmission errors … and will cut resolution or signal if they see them. Many Thunderbolt Displays have flaky or bad "hydra" cables. If your Thunderbolt Display's hydra cable is somewhat flaky, the display might "work" with the 2015 MBP (assuming that MBP is more tolerant of errors), but get cut off by the M4 Mac mini.
A workaround I have seen suggested for a bad hydra cable is to get a Thunderbolt 1/2 cable, and run it from the TB2 side of the Apple TB 3-to-2 adapter into the TB daisy-chaining port on the Thunderbolt Display (leaving the built-in cable disconnected and unused).
I don't know whether a flaky cable is the issue, or whether bypassing the cable will fix your problem, but this seems more likely than the M4 Mac mini being incompatible with Thunderbolt 3 gear!
