In the pilot and first couple of episodes the apartment numbers were 4 and 5. Then they were changed to 19 and 20. Anyone have any clue why they did this?

I think they decided that it wouldn't make sense for them to be able to get apartments so close to the entrance or on the bottom floor. They changed it to 19 and 20 which desn't seem to make that much of a difference to me.

I think Rachel is right. Initially, there WERE in 4 and 5, but due to the "view" that Monica had out her window, I think it was more reasonable to believe that they were on the 4th or 5th floor of the building. When you think about it, given the size of Monica and Rachel's apartment, there would logically be only 4 or 5 to a floor, unless the apartment building was REALLY big. :)