Assuming that Harry and Meghan start a family quickly, I don't think we can compare their situation to that of William and Kate's when they were starting their families. When George was born, William was a part time royal and still working for SAR and then later EAAA. He and Kate were able to have the privacy that they had because of that. Harry and Meghan are in a different position entirely - Harry isn't a part time royal anymore, doesn't have another job, and doesn't have reason to live primarily outside of London. I highly doubt Harry is expecting a similar path to what William and Kate had - even taking out the heir vs. spare dynamic, he's older and he's more established in his role than William was when George was born.
The comparison is more to Edward, Andrew and Anne then William. The suggestion being having a large country home and a small city home. William was never this, both of his are large. The suggestion of Thatch cottage which is at Windsor, and would be a convenient base for a lot of their work. And having a small home in London for work. This is how all of Charles' siblings live and operate.
If they have a large country home, I expect this to be the situation. That the country home is their main home. Windsor makes sense. Its close for London for work. It has good schools for the kids.
If their main home is in the city, which I suspect, I would be expecting a much smaller home at Sandringham.
I don't think it'd be likely that Gloucesters would move into NottCott. But 4B would give them enough space if their children are visiting still. It's still grand as a primary home, but scaled down for the massive Apartment 1. The main reason I don't think Meghan and Harry will end up at 4B is because it doesn't have a private garden for any potential children. Given that their children won't have security assigned to them, I think having a space that's already secured is necessary.
Yes, in no way do I see the Gloucesters moving into Nott cottage. Just because they are empty nesters doesn't mean they will downsize that much
One of the main reasons they lease out Barnwell is because they cant afford to have BOTH a country home and a city home. I doubt the couple is about to retire in the next few years. The Duke of kent would be more likely. The Gloucesters need a London base. And arguably they need one as large as Harry's. They may not need for kids at home, but they don't have offices outside their home. Harry has their offices, which has reception rooms as well for hosting events. The Gloucesters use their apartment not only as a house but for their offices, and for hosting events for their work as well. Nott cottage doesn't have the space for their offices, or for them to host. Not to mention when their grandchildren come to visit.
If they move, I would say 4b. 4b is five-six bedrooms (those counts usually don't include staff space). That is the size bedroom wise of Cambridge's. It is also said to be quite opulent. It would actually serve well for either Harry/Meghan or the Gloucesters in reality. The only reason their current apartment might be better for Harry, IMO is not to make him right next to Harry, but because it seems to have a private garden I believe which would be nice for little kids.