Good luck! I have MBS in a bad way. Every Tuesday I go into the LFS because that is when they get their new fish in. I can't resist a new betta if they have one that is particularly nice. The worst though is when I DO resist for a few days... then give in and go back only to find that beautiful betta was snatched up by someone else while I was debating.
20 gallon divided 4 ways still gives each fish 5 gallons of room... you could easily divide it 6 or 8 ways and then you can go get that other fish.
I have a 12 gallon long and divided it 6 times. I currently have 2 empty spaces but I have about 30 newly hatched fry so I'm doing my best to avoid temptation because I will need all the space I can get to house my babies.