Problem is actually that the root.disk is sized to 1500MB and usr.disk is sized to 3500MB. Wubi needs to override the size from 5000MB to 4000MB in this case instead of trying to anticipate separate virtual disks. This 4GB limit will have to be a limitation for a FAT32 install with the preinstalled image; or otherwise Wubi would have to force download the ISO and install with the conventional method whenever a FAT32 drive is selected in standalone mode.
Problem is actually that the root.disk is sized to 1500MB and usr.disk is sized to 3500MB. Wubi needs to override the size from 5000MB to 4000MB in this case instead of trying to anticipate separate virtual disks. This 4GB limit will have to be a limitation for a FAT32 install with the preinstalled image; or otherwise Wubi would have to force download the ISO and install with the conventional method whenever a FAT32 drive is selected in standalone mode.