What does "his name", refer to in verse four of Psalm 100 in the King James Version Bible? This psalm was written before the word "god" even existed in the mind of man. So that word is not the answer.

The Bible itself will answer this for you. In the back of your Bible, you should have a concordance and dictionary. This will tell you that the title Lord is referring to Jesus. However, in this particular instance, this is not the case. In many modern translations, the divine name Jehovah(the modern translation from the Hebrew pronunciation Yahweh), was replaced by the terms “God” and “Lord,” generally in all-capital letters in English to indicate the substitution for the Tetragrammaton(YHWH), mostly due to a Jewish superstition, in which the belief was that the name of the Almighty was too sacred to pronounce.