Remember that the dimension of the raw casting is pretty meaningless. Those would be machined down substantially from what you have. 4.13" or 4.23" is certainly reasonable to end up with, especially for prototype parts.
From the pictures, those pistons are cast. (You can see mold parting lines underneath, which would be absent in a forging.) This would lead me to believe they are not intended for all-out racing.
XE parts can be pretty much anything. Who knows - Maybe they were dreaming up a low-compression industrial version of the 406 or 427. Too bad the Sheffield guy you got them from didn't know more.