DOT 3 is spec supplied. DOT 4 has higher boiling point and is a little more expensive. Doesn't much matter unless you're racing. I don't tend to use brakes much so I use DOT 3. They are compatible.
Hi Sean
A couple of months ago I changed my master cylinder as my servo was leaking. Replaced all fluid with dot4 very cheap I thought.
5 litres off Amazon for around £20.
Regards
Gareth.