The first thing to note is that most lines are stronger than what the label says. Most 4# is really 6#. Most 6# is closer to 8#, and on you go.
After many years of fishing light line, I think I can land almost anything with 4# line. I've caught carp and catfish in the low to mid teens on 4#. Got a 7lb bass on 2# line.
In really snaggy places, a heavier line will keep you from losing fish and baits, but I still prefer lighter lines.