The initial order was "wrapped". And the cast an crew were sent on "hiatus". Takes a LOT more than two or three weeks to get it all back together.
It's all "filmed" digitally, so no celluloid to develop. The cast and crew are under contract, in case it did well and NBC wanted to extend it to a full season, so it can't take more than 10 days to get everything/everyone back up and ready. Writers were likely told a week before the cast/crew were informed to finish the back 9, so they likely have at least 2-3 scripts ready now. No major set construction/destruction is required, post-production folks are already working anyway, so no big callbacks there. So, seriously, it shouldn't take anymore than 2-3 weeks (a month at most) to get 2-3 episodes "in the can" and ready to air. A 4-month hiatus simply isn't necessary...but nevertheless, that's what's going to happen...