[Shankenstein]

As CCJ's link states, DOT4 will more readily absorb water and other contaminants than DOT3 (which is probably what our cars came with). Also, it's worth noting that most tech sheets for brake fluids (like Valvoline, Castrol, ATE, Motul, etc) recommend that you are careful to avoid mixing different fluids. The chemical differences between DOT3, 4, and 5.1 won't make them immiscible, but if you are tracking the car... the weakest fluid will degrade first.

I wouldn't worry about it too much. Scoop the goo, pull the fluid from the reservoir using a turkey baster or similar syringe, then thoroughly flush the system. If you have access to a powerful vacuum bleeder, I'm guessing that it would help move any gelled fluid stuck in the system.

Racing brake fluid is only intended to be in the system for a year or two... so if you're an infrequent racer (like me), it's best to flush the system for the first big event of each season.

Also, if you're car is seeing cold weather (significantly below freezing), it can cause gelling. Usually the viscosity returns to normal, but some chemicals separate and misbehave at very low temperatures. My company uses plenty of epoxies and industrial coatings, and some have warnings "Throw away if it sees freezing temperatures." because of the separation issues cause permanent changes that lead to clogged dispense nozzles or inconsistent curing.