Help with selecting the correct fly line.

I have a St-Croix Imperial moderate action rod 7'6".
I use RIO WF4 floating line.

The line does not load the rod good enough to cast small streams.

What I need to know is what line options are there.

I have looked at SA Mastery Trout DT4, SA Mastery TRout WF4, SA Mastery Head Start 4 [lines].

I am an *intermediate fly fisherman and have been wild trout fishing for 32 Years. I picked up Fly fishing 15 years ago and do nothing but fly fish for trout.

Any information would be appreciated.