I confess to being somewhat baffled as to why there are so many 1943-dated steel cents with the 4 partially filled in on the dies, and maybe the mint mark, though I never really studied them. The theory that some of the zinc plating was eroding off of the planchets and accumulating in the 4 is quite plausible, though one must wonder why it would accumulate only there and not in the 9 and/or the 3.

FWIW, die erosion across the face of the obverse die is definitely not uniform. In my book on the cents of 1922 I discuss metal flow within a planchet during striking, and how this affected die erosion, at least within the 1922-D cent universe I was studying.

Short summary: During a strike SOME of the metal in the planchet, that which is closest to the collar, flows OUTWARDS to fill in the tiny gap between the edge of the planchet and the collar, as well as moving both up and down and forming the rims on the coin. The upsetting of the rims of the planchet helps facilitate the metal flow into the rims in the dies, though it probably work hardens the edge of the planchet a bit.

SOME of the metal, closest to the center of the dies, flows INWARDS to build up the mound of metal that is Lincoln's head. That metal has to come from somewhere, so the obverse die sucks it in from the fields.

In between is a no-mans-land where it is possible that the metal flows BOTH WAYS, starting outwards towards the collar during the microsecond when the highest parts of the obverse die (those which form the "bay" under Lincoln's chin and the angle at the back of his neck) first touch the planchet, and then reversing back inwards towards the recess that forms the head as that part of the design strikes up.

On the 1922-D and/or "No D" cents, this no-mans-land tended to fall under the 9 in the date and the R of LIBERTY. Multiple dies show these two characters eroded in all directions, while the nearby 1 (one) and Y are basically normal. On the dies used the longest this erosion sometimes spreads to the first 2 in the date and the E of LIBERTY, and maybe the left side of the T.

IF (and this is a mighty big IF) the 1943-dated cents experienced a similar back and forth die erosion at the 4, then the dislodged zinc MIGHT have tended to accumulate there instead of being swept away towards the edge. I offer this as a theory only, and would appreciate any comments. I would also like to know if any of the weak 4 coins also show any weakness near the center of LIBERTY.

TD

P.S.: As for why the 9 was affected on the 1922-D coins and the 4 on the 1943 coins, this might be because of differences in the upsetting of the planchets, differences in the relief of Lincoln's head, the hardness of the steel or the shape of the 4, or any combination of these factors.