Hey there,I built my rack in order to house full grown BRBs. I made it using the Iris VE-175 tubs and melamine. It's now been three years since I built this and I'm really liking it still.
The tubs are sold at Walmart during the Christmas season,and they have dimensions of 4.4ft L x 1.75ft W x 14" H so I would imagine they are probable pretty close in size with your 4x2 cages. I have no experience with using smaller sized cages for adults so I can't comment on that.
I have noted that they definitely use every inch of space in their tubs though nearly 99% of that activity happens between the hours of 1 a.m. - 6 a.m. so I rarely see them out and about. I also use these tubs for breeding and I find it nice to have a large enough cage for both adult brbs to have enough room. Anyway, sorry for rambling, I'm sure someone here has some experience housing them in smaller enclosures successfully, I'll let them talk



-----
Ike Lightner
2.3 BRB (Dugo, Sultan, Roxy, Brazita, & Lucille)