I just bought a FF75 thinking about the small streams of the San Gabriels and San Bernardinos in So. Cal. There's a lot of quiet shallow water in those streams, so I'm more comfortable with a 4 wt line than a 5. I know some FF75s are marked 6 wt, but mine is marked 5 and 6. I'm in Indiana (cold and damp) and I'm anxious to know from those who know if this rod is capable with a 4 wt. I know it'll work out a 4 wt line, but will I feel the sweet loading feel I'm looking for in a 2-number Fenwick.
My experience is that my FF70 does feel nice with a 4 wt. but my FF79 doesn't feel sweet even with a 5 wt till it's got 25 or 30 feet out. Hoping my FF75 can be a nice 4, but even lawn casing doesn't feel the same as on the water.
Whatcha think?

Fish on!
Peter`

My FF75 would not feel good at all with a 4 wgt line. It’s definitely a six in my hands but, rods vary, and casters vary, so give it a shot. The right line on yours is the one that works for you.

" I know it'll work out a 4 wt line, but will I feel the sweet loading feel I'm looking for in a 2-number Fenwick ... but even lawn casing doesn't feel the same as on the water.
Whatcha think?"

hmm, lawn casting or water casting or gymnasium casting etc. the rod loading/ feedback is happening in the air so surface underfoot doesn't seem too important to me. If I can aerialize 15'-30' or 50' of line and false cast it a few times in the yard it is going to load the rod pretty much exactly as it will on the water. The only differences would be in pick-up friction or in rollcasting. I don't think there is much feedback from pick-up and after ~50 years of practice, I still can't roll cast a #4 line effectively.
My thoughts are that for short casts where only 5'-15' of line are in use (rod + leader+ line = 20'-30') a heavier line carries more fly more accurately, and with only 10' of line out even a #10 line would really "load" a "#5" rod. A #4 line at 20' distance is pretty much like casting leader only to me, with any rod.
I like my FF75 with a #5 line at distance >25' if aerial casting or with a #7 if working close or roll casting. Yes I can aerial cast a #4line with it, but the short rod combined with low line mass keep me from mending as well as I want. And the short rod plus low line mass also applies roll casting comfortably with less than #6 line.
But this is more about my inability than the rod's.
I also think that only you can determine what you like. Try the rod out wherever you can with every weight line from #3-#7 and with the casts and mending that you will use most.
I'll say this about shallow water, clear water and trout vs line weight-
The fish and the reel are covered in water for estimate of clarity, water depth where the fish were feeding ~30" with a #7 line last week. I'm not sure what the fly was, #14 Pheasant Dry, I think, but that wasn't the only fish caught there that afternoon and It may have been The Usual.
I'll add that on different days in other years I've taken dozens of parr marked 4-6" trout from that same piece of stream with other 7wt rods.

I've been wondering about--hoping about--that. The streams I'm talking about are shallow and clear enough to see whether I'm scaring fish with the 'splash-down' of a line. I'm, actually hoping that this rod will present a 5wt -- or even a 6wt line -- delicately.
And yes, I need a delicate leader formula. And that means delicate, not long. Do you have any suggestions about where I might look?

That's Peach DT7F in my picture, although the color doesn't show, fish had a lot of color too. I build my own leaders, always have so I can't recommend any extruded leader, ones I've tried had thicker butts than I like.
That water is pretty clear, 6-8" deep where the reel is, and I was standing about 20' from the fish. I guess that if I could see them then they could see me? I'd guess the flow that day at <40CFS, its a small spring branch.
I don't worry a lot about spooking them, they have short memories for the most part, I used to wade right up the middle of strange streams, noting where the fish spooked from and where they hid, then drink a cup of coffee and rig up and by that time the fish would be back on station, I've stood in streams and had adult fish take a station in the wake of my legs, less than a foot from my leg just moments after spooking and fleeing.
carlz suggested lines would work for me on my FF75. I have both and would like use the DT5F more often that the TT6F. I think the last few times I used it with a China DT6F. I bought the China lines as being affordable for experimental line/rod matching purposes, but find myself fishing with them fairly often.

If you don't need a long leader, I would work on presentation casts. The pile cast works well, and being able to straighten the leader in the air and having the fly land first also helps.

If you are scaring fish regularly, it is often from a cast that slaps the water, or from lining the fish. Having fished the letort and other slow spring creeks, patience and a stealth approach are the first requirement, and then no bad casts. Once you put the fish down, it is time to move on.