There are many stories regarding these stampings and the ones most heard (by me anyhow)
4MA have a certain % of NEW iron when cast ie: 15%
4MAB have more new iron ie: 25% bet I many a time have heard that these are tuftrided from the factory.
And then you have the late 351 cranks out of xd/e black motors that some say are the strongest of them all but really I dont think it matters to much as they're all tough as hell and will handle 500+ hp if preped and ballanced correctly.
No doubt some may say otherwise and I may be way off the mark but after 20yrs of hearing the same stories there must be some truth to it