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Originally Posted by Nick  I learn something every day, this is what I found after googling your 4000 thing:
9.4.3.11. Whenever an individual aircraft within a formation becomes separated by more than one (1) NM laterally or 100’ vertically from the formation leader, the pilot will immediately squawk code 4000 with mode C altitude encoded. |
Whenever outside what is known as "standard formation", that being 1 NM and 100'. Non-standard formation is quite common, as are altitude blocks. Inside standard formation, the flight lead has the squawk for the flight. Also, some MTRs require a 4000 squawk prior to entry.