The war isn't really past tense; the swift advance to and battle for Kyiv with the intent of forcing capitulation didn't happen, but as the shift to seizing territory Russia really wants rather than getting handed it by a friendly government imposed on Ukraine has happened, urban warfare has become a thing in Mariupol, and if Russia continues prosecuting the war it’ll probably happen more places.
Other warheads will detonate when they hit the slat armor. With the net result of improved standoff distance. Most warheads don't incorporate enough stand off because it's hard to do so. The extra standoff actually improves penetration.
That said, the AT4 is perfectly usable for side shots, such as in an ambush.
Hitting an enemy fuel truck from 5000m away is a lot safer than running up and shooting it at 200m.
200m shots are common in a city, because cover / concealment is everywhere. Sure, there are still trees and hills to hide in the countryside, but having the option for super long range really helps.
USA sent IIRC 3x as many "dumb" RPGs (the AT4) as Javelins, assuming that the Ukrainians would be fighting in narrow city streets.
Turned out that urban warfare didn't really happen much. But... I think everyone's plan was for urban combat, and then everybody turned out to be wrong. On the USA side, not a big deal, we sent the wrong mix of weapons but we're fixing that with lend-lease / additional aid packages.