Kai'Sa

"Needs to" or you "want it to"?

Because it clearly shows the stacks reaching four stacks and detonating with the following attack in the official video.

If you take Braum and read his passive, you will read the part where it says

"Upon reaching 4 stacks"

In other words, it detonates right at four stacks.

If you take Kai'Sa and read her passive, you will read the part where it says

"Kai'Sa's attacks detonate Plasma at 4 stacks"

It doesn't say it detonates upon reaching four stacks.

It basically says once you have four stacks of plasma on an enemy, you can detonate it with your attack.

And yes I went in and bought both champs just to read their passives in a custom.