I fish for smallmouth a LOT. I also like to use an 8.5" 4wt a LOT.
Sometimes it doesn't work out the way I'd like. When fishing the Delaware for smallmouth, I use a 5 or 6 wt rod, and do alright. Most of my smallmouth fishing is done on Neshaminy Creek, however, and my biggest smallmouth have all been caught there.
Smallmouth in the 16-17 inch range are about max for me, with
12 inch being much more common. Same for Delaware. The issue
I run into on Neshaminy Creek is being able to turn a smallmouth.
Thre bottom is littered with large rocks, and they love to run for cover. The 4wt doesn't turn fish well, but I manage. This measured 16":
That smallmouth ws landed less than 10 yards above this section:
The current flow is tremendous there. The stream narrows from nearly 200 feet wide to less than 30 feet, and it's deep. The trick is keeping them from running downstream. A couple days before Hurricane Irene, I went fishing on the Neshaminy, and hooked into
a HUGE smallmouth. I'd seen these monsters cruising the water above that bottleneck section before, but resigned myself to the notion that they couldn't be caught. WRONG! I was able to get the monster in close enough to know I brought a knife to a gunfight, and he dwarfed the other smallmouth I had caught there previously. As he got clser to the bottlneck, I knew that I was going to loose if he made it into the swift current, so I slammed thebrakes on the reel. He leaped higher than I thogh possible for a fish that large, fell backwards, and spit the little hook. The whole ordeal lasted about 5 minutes, and my heart was pounding. If I had a 6-8wt rod, I might have been able to turn the beast before he reached the bottleneck. Once he reached the bottleneck/swift current, the tiny #12 hook was the next issue, with or without a heavier rod. There was another angler watching, and he said he should have followed the smallmouth downstream
a al' Brad Pitt...LOL!
Conclusion: If you're targeting large smallmouth, go heavier than a 4wt. I would have given anything for an 8wt that day, and even a s6wt would have done the trick. Most of the larger smallmouth I catch dive deep and look for rocks. The trick is having a rod heavy enough to turn them. Casting an 8wt for 2-3 hours sounds a bit ouchy for my shoulder, but I have fished a 7wt for that long.