Question

What is a good level to evolve pikachu in gen 4?

Answer 1

I would say as quickly as possible because the only good move it seems to have is Thunderbolt at Lv. 26.

Answer 2

After level 26, where it learns Thunderbolt.

None of the later moves are of much use:
29 Feint (there are better moves)
34 Agility (Pikachu/ Raichu's already fast and too frail to set up)
37 Discharge (you already have T-bolt)
42 Light Screen (possibly useful, but most ground moves are physical anyway)
45 Thunder (low accuracy, stick to T-bolt)

Pikachu Gen IV learnset

Comments

Light screen and thunder are TMs anyway.

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They're one use before Gen IV, so you might not want to waste them on Pikachu.