He doesn’t have to catch up the full 7-8million votes. Also this time, many people voted to get rid of trump, in 4 years they might not be very satisfied with biden and the trump time will be a bit forgotten. Add to that, Harris as probably candidate may not be seen as a consensus figure.
I don’t think its impossible for trump to win in 4 years.
edit: the Ds can counter the gerrymandering and voter suppression by giving statehood to DC and Puerto Rico, which would increase their tally in electoral college and senators. This will never be accepted by the Rs though.