There are various ways you can check for a dragging clutch; however, a dragging clutch usually affects shifting in all gears. The most obvious case of dragging clutch symptom usually shows up when shifting into reverse. If the shift into reverse is smooth / noiseless then I don't think you have a dragging clutch problem.
Page 13-5 of the 1991 service manual describes adjustment of the shift cables. You can check that adjustment to confirm whether it is correct and if not adjust them to specification. Page 13-4 shows the shift lubrication points. You can lubricate and inspect the bushings to make sure that nothing is broken. A loose / noisy shift lever could be a sign of a gear shift lever mechanism problem. As Drew notes, I think the surface rust on the shift lever bracket is a non issue.
My gut reaction is that you have a 4th gear synchronizer problem or a shift fork / change holder / select lever problem. It is a common shift fork for 3-4.
A dragging clutch is fairly easy to confirm and you can check the shift lever cable adjustment so do those first just to rule them out. If both of those are fine then I would plan for a transmission fluid drain (and maybe replacement). I suggest this not because replacing the fluid will fix the problem; but, because you want to check for metal particle contamination in what drains out and you want to be able to inspect the oil pump strainer for metal particles. If you have what appears to be metal contamination, take a picture, post and ask for advice on whether this is normal or fatal. If you actually find significant metal chips then you can probably fast track to this being a transmission out repair and I would not bother refilling the transmission. If you can't do the repair yourself then its probably time to put it on a trailer and find a competent shop to work on the transmission. If there is no significant metal contamination, you can try refilling the transmission with Honda MTF and see how it works. Fresh oil at the correct level might help with a synchronizer wear issue. Fresh oil will not fix a shifter fork problem.
Good luck.