You can put a heating pad in with them.Unfortunately I don't really have a way to give them a heat lamp.. And I'm afraid of the fire hazard!
X2 definitely introduce them separated by a fence that they can see through.I think they should be fine, but I would be worried about them with the two older ducks. Have they been around each other? We introduced a new duck, who was an adult, to are ducks. They were all the same age and it didn't go well at first. I felt bad having to put them together at night. They may be fine with a group of younger ones. With mine, I had the new duck separated from the other two for the most part during the day and just had them together for short amounts of time. Are drake was pretty mean with her, but we gave away the drake and then the two ducks were fine. Haha. Maybe have them hang out separated if possible during the day and see how it goes. If they seem fine let them all together and see. Then you can judge if they seem ready to all be together at night.