DOT 4 is just a synthetic version of DOT 3. It will last longer before turning into goo, and will have a higher boiling point, which probably doesn't really mean much on these bikes.
Whichever you use, it should be flushed out and changed every 2-3 years. It's also smart to do it on cars every so many years as well, as brake fluid absorbs moisture, and causes calipers to rust/freeze over time, and flushing the fluid helps keep this from happening.
Investing in a set of speedbleeders makes it easy.
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