I’m far from a good caster and have always used the suggested line weights for any rod. However, I have an old tiny 5’ 3" glass Fenwick Ferrulite that calls for a 5 wgt. line. I don’t have one but am wondering what results if I were to get one. Would it be better for use in small woodland streams? 
Thanks.
Ray;
If you have a 6 wt. line try it before buying a 5wt. line. It may slow the action just right for the small streams.
I have that rod and use it with a five weight dt line. It’s a fun rod to fish, especially with small dry flies. I am comfortable with it on all but the biggest West Michigan streams. On the Muskegon or the Manistee I would definitely choose a longer rod for easier mending.
I have one of those that has quite a history. It was found about 30 miles west of where the Teton Dam broke in 1976. I bought it from a dozer operator who found it sticking out of a pile of debris. I think I gave him $25 for it. If only it could tell it’s own story! I use a 6 wt. on it and it does quite well on the small to midsize streams here in SE Idaho.
I confess. I’m getting senile. The sentence I forgot to include was that I wanted to see if a FOUR wgt. line to be suitable. That is the size I don’t have. Wow !
Ray, not to worry about getting senile. As for using a 4 weight line. You can but to get a proper load on the rod you will have to be casting a long way, meaning you will need a lot of line out before the rod properly loads. Personally I never under line but I will often over line. I like to get the rod to load up fast so it is doing most of the work and not me. Larry —sagefisher—
WOW! It sure would be interesting to know the rest of that rod’s story.
Ray, I would put that 4 WT on that glass rod and give it a try. I made a rod with a Fenwick blank which was marked a 6WT. many years ago. I fished it with a 6 and then graphite came along and I put that glass Fenwick aside for a number of years. When I tried it again, I used a 4 Wt with it and it casts great!
You won’t like it at all… just package it up and send it to me. 
I wouldn’t under line it, personally. It will turn into a broomstick and lose a lot of what it was intended for.