Probably a grease filled die. It is an error but a minor one and doesn't really add any value since only 1 numeral is affected. This 1943 is in nice shape.
The grease filled die struck many coins befor they recognized the problem. This probably happened with more than one die. Just a guess.
Well, to be a "Mint Error" (and not a "Mint Variety") it has to meet these standards: A. It occurred at the Mint B. Problems involving the metal of a coin: The metal itself has a defect (e.g. lamination), the wrong metal was used (nickel on a cent planchet), it's the wrong thickness (dime struck of 25c stock), the planchet is tapered, rolled thin/thick, the metal was not mixed properly/improper alloy (cent on brass planchet), the clad layer separated, etc. C. Problems involving striking: the coin is clipped, off-centered, broadstruck, misaligned die, double or multi-struck, flip-over strike, rotated dies, brockages, die caps, weak strike or die adjustments, etc. Now a "variety" means something is wrong with the die itself (unintentional, like a die crack) or the die was changed due to a new design (intentional). It will then stamp what is wrong - or an intentional design change - on to the coin. It exists even if a coin is never struck. Seeing there is grease on a die filling in the 4 which created the illusion of a missing 4, and not a result of the striking process itself, the ghost 4 could be a considered by some to be a variety and not an error. But some can make the case that it was not the die that changed, but something running interference between the die and the planchet, thus it should technically be an error. For me, because the die itself is fine, it should be called an "Error."
There is a lot of confusion for new and even experienced collectors on terminology. For example, error means mistake, and an unintentional variety (3-legs Buffalo nickel) is a mistake, but it is not an "error"; it is a variety. And yet there are many varieties of errors, and varieties of varieties! And some varieties can also be errors! (e.g. An off-centered 1955 doubled die) I find even the TPGs mess up quite a bit on terminology, mainly the difference between blank and planchet. For me, that makes a huge difference. A coin struck on a blank is more rare than struck on a planchet. Several times I have had to return coins because it was not struck on a blank as stated on the label. Even CONECA, supposedly the specialist in error attributions, messes up quite a bit on the differences. People also say "blank planchet" when it should be "unstruck planchet"; it cant be a blank and a planchet at the same time because it can't be both missing a rim yet have a rim.
Strictly speaking, a variety is simple a specific die pairing. So the three legged buffalo is a variety as only a single obverse die had the leg ground away. Likewise, the WAM cents are varieties, the wide letters denote a specific die/dies. VAMS, Newcomb numbers, Sheldon numbers, Overtons, these are all specific varieties. Now, some varieties can also be errors in the sense that the mint made a mistake when they created the specific die, i.e. overdates, doubled dies, cuds, etc., but again even doubled dies are technically varieties as the doubling generally occurs from a single specific die (leaving out those early 70 doubled master hubs). Any other oddity produced by the mint is just a plain old error. So clipped planchets, struck through grease, capped die, broadstroke, etc. These are errors that occur at the mint but are related to the minting process, not related to a specific set of dies.