The even more simpler solution is to just give up on unique identifiers within an airline and simply give each airplane its own name space. The existing system

The even more simpler solution is to just give up on unique identifiers within an airline and simply give each airplane its own name space. The existing system of a 4 digits can then continue, and all that is required is to use a bunch of translation layers to figure out which airline and which plane corresponds to a specific flight. Adding a prefix would mean that all software need to change at the same time in order to include the prefix. Adding translation layers can be done on top of existing old software.