The higher the DOT number, the higher the boiling points for the fluid are. Brake fluid has 2 boiling temperatures: dry (no moisture in it) and wet. The wet temperature is significantly lower. This may cause the brakes not to work when they get hot.
The higher the DOT number the more hygroscopic the fluid is. Most cars are DOT3, which is less hygroscopic than the brake fluid used in motorcycles and some high performance vehicles.
The DOT4 brake fluid is about $7 a container. 3 will flush and refill a GW1800's brake and clutch systems.
Some people wet flush their systems-put the clean new fluid in the reservoir on top of the old and and pump it through until the clean fluid is seen flowing out the bleeders. No air admitted into the system. Some prefer to pull all the old fluid out and then fill and bleed. Either way works well. If the old fluid were dark and overdue for a change, I would pull all the old fluid out first.
It is not a big job. The fronts practically gravity bleed. The rear master cylinder does a great job pushing fluid out.
Its a small, inexpensive job now that prevents a big-dollar one later