SFU1204 or 1605

[ov_darkness]

Thanks, that's what I thought but couldn't explain why.

It's due to the pitch of the thread...

It's not.
If you have a 200step motor it's 0.025mm (25 um) per step on 5mm pitch screw, or 20um for 4mm. Both are fine for typical layer heights. It'd be advisable to use 0.16mm instead of 0.15mm layers on 1204. It'll give you issues with microstepping on 5mm pitch. Eg. 1/16 microstep will be 1.5625um instead of 1,25um.