AS-204 was retronymed into Apollo 1 after Grissom, White and Chaffee died in a fire. The next flight was an unmanned flight of the Saturn IB and it was called . . . Apollo 4?! What happened to 2 and 3?
Wackypedia implies it is because of Apollo 1 and three IB flights went before but that’s now four previous flights counted so then those IB flights would be retronymed to Apollo 2, 3 and 4 and thus flight AS-501 would have been Apollo V.
So why did we go from 1 to 4?
Apollo missions 1, 2, and 3 had been scheduled when the fire during a test destroyed 012 (the spacecraft scheduled for the Apollo 1 mission) and its crew. All Apollo flights were suspended while the fire was investigated.
When the missions were resumed, the original schedule for the 1, 2, and 3 missions had passed. It would have been possible to simply restart the numbering and designate the next mission as Apollo 1. But it was decided to retire those mission numbers and start with Apollo 4.
Which wikipedia page are you referring to? The entry under Apollo 1 seems pretty clear:
(bolding mine)
Earlier thread from 2001: Apollo 2 and 3?. See especially posts #5 and #6 by evilhanz.