DOT 4 has higher boiling points than DOT3, but otherwise they mix OK with each other. DOT 5 is BAD. DOT 5.1 is sorta OK to mix with 3 & 4. Confused yet?

Clutch hydraulic system doesn't get as hot as brakes, unles you got a turbo sitting right next to the clutch slave. Use either DOT 3 or 4. But it's usually good to bleed enough fluid thru the system so it's all fresh. That takes care of the mixing worries.