The most difficult rod to line that I have is a 4 wt 4 pc I built on a H&H IM6 blank. After trying several 4 wt lines on the rod (DT, WF and different brands) I tried a 5 wt...I could tell the rod was over lined, it seemed to bend to deeply and had a hard time dampening.
I came across a Joan Wulff signature series TT5...the line is different that regular TT line, orange and white...Both JC and Ron Kusse cast this line on a 5 wt rod and both agreed the rod was underlined....I believe the line is a 4 1/2 wt or there abouts.
The line works well on the 4 wt H&H, it seems to be made for that rod. Although the rod will never be a fav of mine it is now at least fishable. And being a 4 pc...I'll keep it.