inconsistent STI indexing I have an 8 speed system with 12 year old Sora STI's. Other components of system have been recently replaced.

All shifts up and down are fine except that 4-3 does not shift, only produces gear clicking noises.  The slightest extra touch of shift lever completes the shift.  Attempts to adjust cable tension to resolve always affects other shifts adversely.

Has anybody got any suggestions on likely cause please ?
The first diagnostic step from where you are is make sure the little screw on the front of the lever is snug, then disconnect the cable from the RD, pull away the housing from the lever, confirm the cable is intact and not jammed/fraying inside the shifter, and gently tug on the bare cable as you shift back and forth through the gears. If it feels normal there, including the 3-4 shift you're having trouble with, then the shifting problem is of the mundane variety, i.e. excess cable friction, hanger alignment, tweaked derailer, improper setup, etc.

If it isn't shifting correctly even as you're tugging on the bare cable, you need to figure out if it's a broken spring or other internal part or if it merely needs cleaning and re-lubrication. It's usually obvious if something is broken, but if not you can do the clean/lube step first. There are different ways of doing this, but the quick and dirty one is just flush it good with silicone spray lube, working the pawls back and forth with a pick if they're accessible, and also putting a dot of grease on the pawls if possible in emulation of new STIs. If it's not broken that will make it work again about as reliably as anything else. Alternatively, a more perfectionist approach is unscrew it from the mount, take the hood off, and put the whole thing in an aqueous parts washer without agitation, and then re-lube in a similar fashion as above. 

If it's broken, you should probably throw it away and get another one. If you want a really questionable new hobby, you could try to track down a donor to steal parts from and do this. 

The good news is that just getting another replacement used right ST-3300 or new Claris 2400 is pretty feasible and cheap. If you get another one of the exact same shifter, you can play the trick of unscrewing the old one from its mount and putting the new one in its place without even having to redo the tape or brake housing.