The DOT 3 vs. DOT 4 discussions get beaten to death. I think that some people have a biased opinion that DOT 4 is always "better" (maybe because of the higher number,LOL?). They both work. They both have pros/cons. Choose the one based on your bias. Some choose DOT 4 because the think they need a higher boiling point. I choose DOT 3 because that's what came OEM in my ride AND I think DOT 3 manages moisture better (my bias). I live in the flat lands and have never put my brakes into a high stress situation.
To ease the OP's decision, I recommend that he follows what's on his reservoir cap (DOT 3). That's probably the OEM fluid. The manual says you can use either. I'm just trying to ease the OP's decision making process, but he did not share with us what his driving conditions and style are. So, around and around we go!