Didn't see it in the patch notes but I know they do not always say everything and I am on a new play through and am not high enough to test it out...also its a bit tough to test out but I would try if I was high enough level. Does anyone know?

Partly. Apparently the damage increases but the AOE of the explosion when you take rank 4 is reduced back to normal instead of increasing.

I have rank 4 for a long time already. But I did not notice that something is wrong as I don`t use grenades too often ( because of settlers and companions).

Is there a way to check it? I have no idea how can I know what AOE is normal for rank 4 and how can I test it.

true story. I don't know either as you have had rank 4 for a long time. As soon as I am able to get to rank 4 I will save right before and see if I can see the difference. Its hard to test was hoping a modder type user would lol

Molotovs maybe? If the expanded area of effect is working as intended, then the flame pool should be bigger. You'd probably have to use the console commands to add and remove rank 4 of the perk to see how it compares to rank 3.

I never heard of this perk being bugged only the Big Leagues.

google it. Its something with the AOE not the damage.