Is there some part of my previous post not getting through here...? The towns are not locked for disc space reasons. All field files are stored on all discs, and disc 4 actually has some extra scene data on it (which I haven't examined yet). The locked towns are more likely a result of development time cut short and/or not wanting to write extra dialogue for all people during time compression (ironically, adding the extra handling for disc 4 during time compression would make the field data larger, but the effect would be very marginal). Again, all discs contain an identical set of field data files, that is all script for all discs are stored on every disc.
EDIT:
The extra field data on disc 4 is actually just a sound file of a chorus echoing "FITHOS LUSEC WECOS VINOSEC" accompanied by something that sounds like a blowtorch.