Brake question.

Alright, last night my brake light came on, figured brake fluid was low. I was in a hurry and forgot to check whether it took DOT3 or DOT4 fluid. I figured DOT3 because I don't have ABS and I've got the drums in the back. I poured it in, took a look at the cap and went *uck! I saw DOT4. Is there anything potentially harmful about putting the wrong brake fluid in. The brakes aren't acting any different yet. I am planning to replace my master cyl. soon as it leaks and doesn't keep consistent pressure.

Any thoughts?

I tried a quick search on yahoo with "dot3 vs dot4" and got a number of hits. Didn't read them all, but I got the sense that they mix OK. You might try yahooing/googling yourself, and see what you think.