However, you can use DOT 5.1
"Since DOT 4 and 5.1 are both glycol-based brake fluids they are compatible with each other, which means they can be readily mixed without harming your brake system. It is important never to mistake DOT 5.1 (glycol-based) with DOT 5 which is silicone-based and should never be mixed with any other DOT fluid."
http://www.epicbleedsolutions.com/resources/faq/difference-between-dot4-and-dot51-brake-fluid/