Steel, four of them, and light — lighter than most players expect the first time they change a set. On a short scale with a small top, gauge is not a tone preference so much as a structural decision, and going up a gauge to get "more sound" is the most common way to make a cavaquinho sound worse.
Why light
Tension rises quickly as gauge increases, and a cavaquinho top has very little area to spread that load over. A heavy set pulls the top, chokes the response and makes fast strokes harder, which is precisely the opposite of what the instrument is for. If you want more volume, that is a right-hand and setup question long before it is a string question.
Steel dies quietly
Strings on this instrument wear fast because they are thin, struck hard and constantly. They do not usually break; they go dull, and the dullness arrives gradually enough that you stop noticing it. The tell is that figures from the batida pages stop cutting — the attack softens and everything blurs together. If an instrument has started sounding tired for no reason, change the strings before you conclude anything else.
Changing them
Change all four at once. A single fresh string beside three dull ones is louder and brighter and makes the instrument sound uneven in a way that is worse than four evenly worn strings. Wind them neatly — a small headstock leaves little room, and messy windings are a tuning-stability problem rather than a cosmetic one.
They settle fast
Unlike nylon, a fresh steel set settles within an hour or two of playing. Stretch each string gently by hand once it is up to pitch, retune, and repeat until it holds. After that, drift is a tuner problem or a nut problem, not a string problem — which is where the setup page takes over.