DOT 3/4 and DOT 5 are not miscible. But they do not react with each other. The glycol and the silicone oil will form two separate layers if mixed. That's all.

You want to get all the glycol out that you can, only because any that is left will gradually attract water and cause corrosion where it sits.
DOT 3 & 4 also have a lower boiling point than 5, so any glycol left in the system will boil first