On small streams I use a graphite 7 1/2 ft, 4 wt rod and I think that is a very good rod for that type of fishing.
For both roll casting and all other types of casting. On small streams I use sidearm casting a lot, because it keeps your cast low, under the tree branches. You should learn to cast sidearm on your backhand side as well as your normal side.
The action of graphite rods varies a great deal from very fast action (stiff) to slow action (bendy) and a whole range of medium.
You just have to try casting a lot of rods, to see what works for you.
I've never overlined any fly rod. I have a 7 1/2 ft rod rated for a 4 wt line, and it casts well with a double taper 4 wt line.
If I test cast a 4 wt rod and it didn't cast well with a 4 wt line, I would just not buy that rod. I wouldn't put a 5 wt line on it to make it work.
When you're spending good money for a fly rod, you shouldn't have to do that. It should cast well with the rated line. If not, don't buy it.