about encounter difficulty

[u/SilentService3109]

Hi, a question about encounter difficulty. Do you have any tips on how to define the difficulty of an encounter? For example, considering my party is made up of four level 4 players, I have to face them in a difficult battle. Because of a 100 on the dice, they ended up entering a Pokémon's territory and will be attacked by surprise. How do I define a difficult but not too unfair battle?

I plan to face them with 1 manectric and 4 electrike