It doesn't say spirit in my Strongs Concordance
Good catch and you are correct. KJV adds spirit but for good reason, I don't always agree with KJV when looking at the literal Gk but in this time it is clearly correct. Young's Literal Greek Translation says this.....
1 John 4:3
and every spirit that doth not confess Jesus Christ in the flesh having come, of God it is not; and this is that of the antichrist, which ye heard that it doth come, and now in the world it is already.
So it is saying that every spirit (man, satan, angel, etc) that does not confess Jesus in the flesh is that spirit of antichrist.
antichrist, means "against Christ". Simply that. No where is it used as a personal pronoun that I can find. So we have to look to other scripture that does not use the term to try to find this person, if it is relating to one person.