Firstly, I have checked a similarly titled post (Honda CB400 Super-Four - Power Issue) and their symptoms are different so it's not relevant to me. My bike is a '97 CB400SF, all stock except for instrument cluster and exhaust (i.e. original airbox, filters, carbs, jetting, engine). No change in sprockets or gearing, either.
I got it modified from original and it started behaving differently shortly after but due to work and family commitments I had left it unaddressed for abot a year. It starts and runs fine, but appears to hit redline very early, requiring premature gear changing (when compared with any other CB400). Because there's no tacho installed I can't estimate rpms, but it definitely sounds like redline because I can bounce off redline before changing gears. No spluttering, struggling or smell of unburnt fuel, either.
Not sure where to start with this, but want to have a better understanding before I inevitably send it to the mechanic for better diagnosis and fix (I don't want to leave the mechanic to work it all out himself, it may cost me more than the bike is worth). Plus it's a good opportunity to learn more myself.
At the moment, top speed in 6th is about 65mph. To bumble about city streets it's 3rd/4th gear.
It recently went in for a regular service, so had a carb clean, jets checked, new oils, etc. Partly because I wanted a clean slate to try diagnose this, and I didn't ask the mechanic to specifically address this because I'm in a new town so don't have a trusted mechanic just yet.
Any recommendations where I can start to at least help diagnose the problem? It's likely out of my skill set but at least knowing how to diagnose more problems is a skill I'd like to develop.
One thing I want to check is whether all 4 cylinders are firing. I believe so but I haven't specifically checked. Easiest way I can think of is just checking if all 4 exhaust outlets are hot. Presumably so if the mechanic's aforementioned service was done properly!