DOT 3 and DOT 4 are glycol ether based. You can mix DOT 4 with DOT 3 brake fluid, but never add DOT 3 to DOT 4. DOT 5 brake ...

When topping off brake fluid, refer to your manual on which brake fluid to purchase. Be sure to use a funnel when adding brake fluid. If it spills, it needs to be cleaned up immediately.