DOT 3 and DOT 4 ARE interchangeable. DOT 5 is a synthetic fluid and is NOT interchangeable with 3 and 4.
The only way brake fluid could be getting "burned off" by your engine by way of the booster, would be if the seals at the back of the master cylinder, where the pushrod goes thru from the pedal and the booster, are leaking and allowing fluid to enter the booster. Pull the vacuum line out of the booster and look for fluid residue in or around the vacuum line.
Also, have you looked very closely for any stains or residue at all four calipers?
I'm betting your master cylinder is shot, although it would be odd to have fluid leaking into the booster, I can't say that I've ever seen one leak that way before.
1995 YJ Rio Grande