I’m about to get into Quake mapping. How similar is TrenchBroom to Doom Builder?
From my budding experience with TrenchBroom, it works nothing like Doom Builder. Doom works off of sectors, Quake works off of brushes, convex polyhedrons that form the many faces of a level, usually in the form of cuboids when you’re making rooms 'n stuff.
There should be an option to turn on all four viewports somewhere in the options. Do that as soon as you can, it will REALLY help you in the long run.