It is normal and not rare for a 1943 Lincoln Steel Cent Wheat Penny to have a missing or weak 4 or other date digit. Usually this is caused by a greased filled die cavity that doesn’t allow the digit to be struck properly.
Another theory is the 4 wasn’t engraved in the master hub very well and that resulted in the 4 being weak. The latter rather than the former makes more sense.
It is a minor struck through error but it is too common to add an value to the coin. A few call it a Weak 4 to make it seem more valuable but such is not the case.