How to remove about 1/4" of wood from in-place beam?
The most popular tool for the job is a power planer, which you're unlikely to own unless you hang tons of doors. The equivalent hand tool would be a bench or jack plane. Overall hand planes are more useful, but high quality ones have eye popping prices. 

The most common term would be planing (i.e. to plane). Though just to make things confusing the machine for one side of a board flat is a jointer, while the planer makes the other side parallel to the first. So in wood working (as opposed to carpentry) the operations are called jointing and planing. Most people call portable planers "lunchbox planers" because of their appearance, and stick to power planer or electric planer for the handheld versions. That's not universal though, so expect some crossover results from search engines.

Also if you should ever be in a situation where you need to lift a lunchbox planer use both hands and lift with your knees, not your back. That name is extremely misleading; they weigh a ton.