Strictly saying it's "too big" for a mid-sized tower can be a bit misleading. There are a lot of different sized/designed cases out there that fall into the mid-sized tower range. I'm sure some of them could house one of these beasts.
Take my CM HAF XB Evo case - it's not a mid-tower, but it can hold a decently sized GPU without too many constraints. Max GPU length is 13.1". Depending on what else you may have inside, it could restrict how long of a GPU you can use, but there haven't been many GPUs that wouldn't work in the case except for the high-end AIBs for the 3090/3090Ti cards that are 13.5-14.5"
If the 4090 is as long as those high-end AIB 3090/Ti cards, they'll be pushing 14.5" (36CM) or longer and that's just nuts. If this is the case and I had to have a 4090, I'd have to move back to my full tower that's tucked away in the storage space in my basement.