I haven't fished the high-speed C4 (just the low-speed models in bass and steelhead sizes), so I can't compare it to the C3. However, there is an extra bearing in the C4, so it might be a little tougher, depending upon where the bearing is. It should be smoother. Outside of the differences in the bearing and the gears, the reels should be about the same in designed strength.

I know I do like my C3s in 6500 and have had no problems with them. Wish I could offer more insight.