Great choice making a rack, youll be breeding in no time
As to your questions, you actually did better by getting younger rats. Sometimes the petstores will sell you breeding sized rats, and it turns out to be they are too old to breed or getting close to it.
It seems like they are a little over a month old, seperate the males from females and introduce them at three months of age. Earlier if they are big enough by then to handle breeding, but I like to wait until they are done growing.
1.4 could become a little crouded should all four have babys at the same time ( Average 14 babys x 4 = 56 babys! Ive had more than that before) I like to do 1.3, but 1.4 could work you just need to make sure that it doesnt get packed.
I agree you did good getting young rats instead of full grown ones. I also agree 1.4 is probably too crowded if you plan to let them give birth in that tub. 1.3 would be better.
Anyways, just give it a few months and you should see babys, watch your males because sometimes they can be duds, and refuse to breed for some reason.
hope I have helped!
- Matt
PS. google helped me ALOT when asking certain questions like growth rate and development.