The TRX450 Foreman motor has 432cc cast into the jug. Is that what you are talking about? The 400 and 450 motors are very similar, so someone might have swapped a 450 motor into your frame, or swapped a 450 topend onto the 400 motor..? Which year is your TRX400?
Look under the seat on the upper frame there might be a model sticker. On the front of the frame you should find some stamped numbers. If those are missing you might have a stolen bike...
A 1995 Foreman had a 400 motor but the 98 & up 450 top end could have been swapped onto the 400. If your cylinder and piston measurements are accurate your TRX450 cylinder and piston are both worn out well beyond the service limits. See the two attached images below and/or find a copy of the service manual HERE.
It doesn’t matter if the machine is a 450 or 400 or FW or FM Or FE or S or ES or anything like that. The only thing that matters is that 432 stamped on the jug, it’s a 450 jug so you need a 450 piston and by your measurements your going to need to go oversize so either take the cylinder to a machine shop and ask them to figure out what oversized piston to buy or replace the cylinder with another one that is either new, or refurbished.
I’ve said it before and I’ll repeat myself once. Everything about the 400 and 450 engines are interchangeable. The bottom ends, the jugs, the heads, all swap. It’s common practice to put 450 jugs on 400 engines for more power, so I’m not even the littlest bit surprised to see someone with an old 400 find out their machine has a 450 jug and piston in it.