When people say will my cat(s) get on with a new cat? I always think this, do you like every one you meet? No you don't and neither do cats. That said your cats are still young and have been used to living with one another. With commitment to gentle socialzation it is highly likely you will have four very happy cats.
You will need more cat litter trays, golden rule one litter tray per cat plus one extra (so you'll need five trays and in different private places in your home).
You'll probably need to provide more places for them to nap and explore. (This doesn't have to be expensive, a cushion on the top of a book shelf, a draw left open to curl up in etc)
Other than that go for it ! I don't see why not !!
(but let me just stress again GENTLE SOCIALIZATION )!!!