unless you have flushed out ALL the old brake fluid, I wouldnt put DOT5 brake fluid in...
DOT5 brake fluid is silicone based, as opposed to DOT4 being Glycol based: the two are not interchangeable/cannot mix.
Also, DOT5 is not rated for any kind of Track use due to its lower boiling point (in comparison with DOT4), so if you plan on doing a lot of heavy braking or track work, I would stick with DOT4
Unless of course you mean DOT5.1, which is a different beast entirely and probably is a worthwhile upgrade (DOT 5.1 is also glycol based, and so can mix somewhat with DOT4 just to add to the DOT4/5 confusion!)
You can find a pretty good explanation of the various DOT brake fluids from Opie Oils
HTH
Mick