If you are a cat 4 you can race A, B and C (in your example) and score points in all three races. For example if each field has 70+ starters and you win all three you’d receive 33 points.
Unless the promoter states something like “race together score separately” you will race every other category in your race and be scored together. For example 70+ starters and you are a cat4 in the A pack and finish 5th place. You’ll get 7 points no matter if everyone in front of you is a cat 1, 2, 3 or 4.