Absolutely, but you need to insure that they are male and female. This age is still too young to get a reliable sexing using probing due to the imature sex organs. The best method is to take and submit a blood or feather sampling to a licensed veterinary laboratory.

Then, separate the young birds to ensure that you keep the male and female together and away from the other two in the event that you would accidently get them mixed. If you are going to keep them all together, you might want to sex all of them to insure that you have a male and female. Let's say that you have 3 males and one female. The dominant male will bond with her leaving the two brothers to bond. You won't have to decide for them, nature will take care of this pairing for you. It is possible in this scenario if you do not want an additional pair of breeding swans to keep them on the same lake since they were hatched and raised together. Good luck. The Regal Swan