You can divide whatever tank you want! dont forget bettas need quite a lot of surface area though because they breathe air. I know one person that has a 3' divided into 12. So a betta gets 3" x 12" x 12" = 6.75L which is 1.5 UK gallons each if the tank was fully filled.
So if you had a 4' x 12" x 12" you could have 16 bettas on that principle.
If you need any ideas for making dividers pm me, I've been making them on the cheap too.