in the first episode the starters were taken by gary and two other trainers leaving pikachu for ash, making 4 trainers from pallet town, in the second episode, officer jenny tells ash that four trainers came to viridian city before him, making 5 trainers in viridian city, then in gen 4 we have paul who chose turtwig then went to a region that wasn't sinnoh, and has a torterra now, my theory is that paul is the 5th trainer in viridian city