i thought that the author should have pushed for the number five, but then i realized that the result of this scheme would make for the fifth sister. i don't know if i accept this creative choice. the plot itself is asking the readers to ignore a lot of how this whole scheme works, even accepting the premise that fran can actually revive the specimen with the desired parts.
the sister who lost her legs still has a chance. actually, there must be enough parts to re-assemble the rest of sisters out of the remaining pieces (not going to bother puzzling it out). it's just that the legless sister doesn't need to be mixed with the others.