system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon Anime] Why did Prof. Oak arrange for 4 trainers to pick their starters in the same day when he only intended to give out 3?

He only gave Ash Pikachu after Ash insisted on taking whatever Prof. Oak had left. So he was just planning to give out Charmander, Squirtle and Bulbasaur.

What would have happened if all 4 trainers showed up at the same time?
gpt
It was clearly a first come first serve situation. Oak is a busy man, he has to care for all the Pokemon that his trainers catch, continue his research projects, and build an audience for poetry lovers.

Oak was already screwing with expectations, since the trainers would only get to pick what was left, rather then of all three options.

It's possible given that starters are bred and raised by specialists, that oak had made his requests unaware that there was more then three kids coming. One of the kids may have just turned 10 years old the other day, or their family just moved to pallet town which would throw off his carefully curated plans.