I think the 750 and 400 calipers are different. I'm not sure though. I suppose you're thinking of buying one from a picture? If you have both you can compare them side by side, there's no magic... if the bolt holes and pistons align and the things are the same size they are most likely identical. If you're talking about a CB400F1: all CB400F, CB350-4, and the disk brake 350 twins (CB350G?) used the same caliper. The 80's and 90's CB400F SuperFour and CB-1 are totally different.
Check a parts site to see if the two models have the same caliper part number if you don't have both. Different number = different part, Honda used a lot of carryover parts and never used a new number for an identical part for a new model in my experience. There are "non-400" parts on a 400 here and there, the "model code" middle 3 numbers in the part number identify the first model a part was used in.
