You can't put 4 adult BN's in a 2ft tank. 2 max. You'd need to rotate females one at a time and have the other 2 in a seperate minimum 2ft tank.
They will breed regularly - if you had 3 females you could expect up to 300 fry at any one time if all 3 laid eggs at once (which is possible)
Common BN need to be about 1.5" before LFS will take them and this means at least 8 weeks of feeding quality foods and probably at least 3xweekly water changes as a minimum - I would also consider BN too messy for sponge filters.