Start with 4 A-levels then if you find it hard or you don't like one drop down to 3. Don't jump the gun and drop one before you even know what the subjects are like at A-level.
This sounds like a vaguely prattish thing to say but I think the difficulty of studying 4 A-levels is often overplayed on here. (as in, it's a fair amount of work but it's not an absolutely herculean task for a lot of strong students) Plenty manage it, particularly when one of them is further maths, and the 4th subject might provide appreciable breadth or a safety net. Since the way admissions are set up in this country means that if your offer is A*AA and you get A*A*B because of a bad exam, you might be stuffed, and this situation could be possibly be avoided with an extra A-level. Just something to add into your consideration.
I don't know anything about the welsh bacc though.