You can use #4 for the loops,but I mainly use #4 for my dacron endless loop strings.When I make a low stretch HMPE type string I prefer to use BCY 1D serving .019 for my endless strings.It's alot stronger the #4 nylon and works great for end servings.But you can still use #4 nylon for HMPE type strings if you would like,I just prefer 1D for my endless strings.I'll even pad the loop ends on some of my HMPE endless,I just run exra strands around the loop posts before I serve them up.Like say on a 10-12 8125,you only have 5-6 for the ends,pretty skinny.