Just handwaving the solution, but your nostr client would keep sending 4 sat zaps every minute to the seeder while checking that the file is still available and downloadable (somehow?). When the seeder notices zaps are no longer sent, they can decide to stop seeding.

This is somewhat similar to how value4value is implemented in podcasting - like in Fountain or Breez, the difference is that there the "stream" of sats is voluntary, not in expectation of service.