@Tricky
Basically, for your "4 banks of chipram" approach to work, you'd have to have 4 completely independant instances of AmigaOS running, 3 of which are suspended.
The problem I see with this, is DOS. Imagine if you swapped instances while someone was accessing the disk. In the new instance, you delete some files, maybe even files that were open on the other instance.
Whoops! You've just corrupted your hard disk.