4-cars are useless for the duties the existing 318s and 320s work as platform lengths are limited to 6 x 20m. Far better off with a 3-car fleet that can be multi'd up if needed.
Not exactly. The stuff to make the 321s into 320s included, but was not limited to, taking a compressor of the 'spare coach' and bolting it onto the underside of the pantograph coach. The rest of the stuff was rather superficial, not dissimilar to a midlife overhaul.
The only reading this work was undertaken was to make them three coaches long, so a uniform fleet length was achieved.
You could quite easily do all the other stuff, but leave them as four coaches. This wouldn't be uniform, but would give you about enough units for 8*23m trains to work East Kilbride. Granted, the diagrams would need redone as they currently interwork with the Kilmarnock and Barrhead trains, but not unachievable.
Yes, it would be easier from a timetabling perspective to have a uniform 6*23m length across the Strathclyde suburban routes, but orders from up on high seem to think that East Kilbride warrants the extra capacity (I cannot comment, as I've never been).
Wether this happens or not is simply if it's cheaper than buying a new fleet. ScotRail might already be looking at replacing the 318/320s in which case its razorblades, but if it's cheaper then we might see these kicking around Strathclyde for a while yet.