OTS is very real for all tanks. Essentially, there are several aspects, For a breif summary, you have the concern of TDS buildup from evaporation and replacement of water without the removal of any of the solids. You also have a concern with your buffer being consumed by natural process and then a subsequent Ph drop in the tank. Lastly you have a buildup of pheremones and other chemicals from the livestock in the tank. The fish in the tank acclimate slowly as these changes take place, but adding new fish is usually deadly for the new guy, and doing water changes rapidly changes things in the system so your fish can't acclimate. a 10% shouldn't have made that significant of a difference but it may have so I wouldn't rule it out totally. Either way, very very frequent low volume water changes should bring back the tank parrameters without shocking the fish.
The thought on my mind is did you get any funny smells? Is it possible that aneroebic pockets formed and you released some nasty gas into the tank? 4 months without a cleaning could easily cause OTS, but could also allow pockets of some really nasty sutff to form in your substrate.
dave