Yrden sign. What is he doing? (The Witcher 3: Wild Hunt)

PS I can't set 4 traps and the trap only fires 1 lightning bolt at a time.

The answer to this question lies in learning the game in the signs section. And the fact that 4 traps are not set for you is your problem :)

the second mode for signs is to keep the button pressed

RedBlackDish
Some on the forums said that this is a bug and now hope for a fix.

procherk
I can't put more than one trap, and it was clear from what I wrote before

It's a crooked translation.
The trap is always 1-a, just, depending on the pumping, it can "work" on 1-2-3-4 targets (ie - "4 charges" in one trap) at the same time. If, of course, the goals are within reach. And yes - "lightning" is not cumulative - 1 target = 1 lightning