The division between seasons of Pokémon is based on the openings of each episode, and may not reflect the actual production season. The English episode numbers are based on their first airing in the United States either in syndication, on the WB Television Network or on Cartoon Network. (Other English-speaking nations largely followed either this order or the Japanese order.) Subsequent airings of the English version follow the original Japanese order, except in the case of episodes which are no longer shown in English.
When aired in the US, the first couple of episodes of this season were aired as part of season 4, but continued to use the 3rd season opening. The 4th season opening did not start until later.