the reason you cant order is because it is out of stock. but if you google “4 in 1 cherry tree", you will find more webites. Some websites say that the tree can be shipped anytime, but some say it has to be shipped during winter-spring (then you have to wait).
Stella cherry is a very popular sweet cherry and doesn't need a pollinator, hardy through zone 5. 4-in-1 could result in the dominance of one type over the other three, thus need some kind of maintenance.