What is the value of a 1943 steel penny it is missing the 4?

It depends on the error. If the coin is damaged and the 4 is missing, it takes away value of the coin. If the coin isn't damaged its possible it is a grease filled die, a minor error that adds only cents to the coin's value.

If worn/damaged the coin is worth about 2-3 cents. If uncirculated the coin is worth about $2-3 with the grease filled die.