FishinGuy
 I never had break off issues with 4lb using a 10'6 Lt slow rod and a reel with a good drag. Always check the last few feet of any line for abrasion after landing a fish.

Well I have seen my 6lb main line mono go from perfect condition with absolutely no abrasion to all scratched up and nearly broken off after landing a single large hot fish in low rocky water.  Those shale rocks can be very sharp in some locations, and it takes more to break 6lb mono than it does 4lb flouro, that's just a fact.  Guess everyone's experiences are different;  But I know I have landed fish where my 6lb main line mono was hanging on by a thread, and I wouldn't have landed those fish on 4lb flouro main line.  It's always the really large fresh fish too.

No doubt shale will chew up your line in a hurry. I was simply sharing my experiences. I certainly broke off a few fish. But as far as I can remember it was always a knot failure. We may just fish in different areas and I may chase fish down stream more readily than you.