X on front of block is Mopar Performance block. It came before the R blocks.
I had one that was 4.100. It later got bored even a hair more by new owner.
Even those, i have never heard of anything past 4.125
I think the poster above questioning the measuring tool is in to something. I'm a 3D drafting engineer, I've seen even good brand micrometers, especially digital ones be off by as much as .25 of an inch. Especially if they've ever been dropped. We have ours calibrated every month and replaced twice a year for that every reason. But that's because we require a tolerance so tight, most machine shops can't do the work. We get down to .0000008 tolerance. Standard micrometers are certified normally to .0001 or the cheap ones even less