To use or not use DOT5 fluid?

jimjimjimmy said
dot 5 will mix with dot 3 and dot 5 is top notch

Hi Jim,
You mention DOT 3 will mix with DOT 5...is there a problem with DOT 4 compatibility? I am asking to gain knowledge and I don't know what DOT # PO used. To be safe I will flush out system...
Thanks , Greg