Question

How come I found a pichu on route 4 in ultra sun?

Comments

Are you sure you found Pichu on route 4?

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we’ll need more context. what time of day was it, did you have any roto-boosts, have you experienced glitches before, have you double checked?

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Why would time of day and roto boosts impact anything?

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Some Pokémon only appear in the day or night.

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Not Pichu though. Pichu only spawns on Route 1. This is either a glitch or the asker hadn’t checked what route they were on.

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Either he got confused or there was a glitch because Pichu is not found on Route 4.

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Yeah, that’s what I’m saying.

Answer 1

As seen here: https://www.serebii.net/pokearth/alola/route4.shtml that isn't possible.
Are you sure you didn't mean route 1? https://www.serebii.net/pokearth/alola/route1.shtml