I thought we covered this already...

Under this scenario you *could* just generate the map from the top block, but you are not *forced* to until the block you are working on becomes too old.  Yes, your submitted work would include the hash of the block you think is top (as normal) as the previous block reference, you just wouldn't be forced to use this most recent block as map seed.  As such, forced map resets would be 4 times less frequent.  We could use any number for the allowed depth, really, and could even have this number adjust dynamically based on block rates, to keep map refresh frequency relatively consistent with network hash rate.