It has also been my understanding that both DOT3 and DOT4 will attract water in the system and when the color of the fluid changes to dark it is time to change it all together. I know there is a recommendation about mileage as to when to change your brake fluid, but as in most things I have found there are no absolute.
It's not absolutlety necessary but it's a damn good idea (to use DOT4).. Wal-Mart has Prestone DOT 4 for $1.99/bottle. I use DOT4 in my bikes and cars because it lasts longer and has a higher boiling point.
Yes, they (DOT 3 and 4) can be mixed. I have used both in the same vehicle. I'll buy and use a bottle of DOT3 if I know I'm just going to pump it through the lines and out of the car or bike and then fill I'll fill the lines with DOT4. If your just bleeding the lines on a VN750 then it should take about 30 mins total for the job if you gravity bleed, which is what I do - just let a "healthy" amount run out of each bleed valve into some clear tubing into a container.
DOT 4 absorbs less moisture form the air. I also try not to bleed the brakes on a humid day.
DOT 3 and DOT 4 are functionally very similar. 4 has a higher boiling point, but motorcycle brake systems don't generate the kind of pressure and temperatures that need it on the street, in general, although it certainly won't hurt your system to put it in. DOT 3 and 4 are fully compatible and you won't have any problems mixing the two. In fact, many bikes just say DOT 4 or 3. The difference is that DOT 4 has a slightly higher boiling point, and is thus prefered. Shouldn't make any difference for real-world riding though. Now... DOT 5 is very different
--it's silicone based, doesn't absorb water, isn't corrosive, is bad for some seals, is hard to bleed, and is not mixable with 3 or 4. Stay away from it--it needs a system designed for it. And it comes as stock item in Harley's, so it must be terrible. :->
Dot 5 is synthetic...you can't have any old fluid present...and the bikes seals were not designed for dot 5 use...it may destroy them. So please...use dot 4. Saying that, yes Dot 5 is great stuff...but the system has to be designed to use it. Race cars use it..but they were made to.......
brake fluid is silicone-based and is used primarily by the ARMY for cold weather conditions more than anything else. It absorbs atmospheric gasses which can cause bubbles at higher altitudes. Dot 5 fluid does not mix with water, so any water that gets into your system will sit in the bottom of your caliper and corrode it, instead of being put into solution where it could be mitigated by corrosion inhibitors.