I have absolutely no experience with owning finches, but I do know that waxbills are large insectivores and should have mealworms as a supplement to their seed, and that they appreciate green food.
Also, finches tend to pair up and I'm worried if you get three males and one female that they will all fight over the female come breeding season, which could make things complicated.
Apparently, like canaries, they need a high calcium diet, especially females, as they are prone to egg-binding (this can happen regardless of whether or not they're breeding). Doves are like this too, so we make our dove a high-calcium grit with mealworms that's basically finely crushed mealworms and calcium mineral blocks.
That's about the extent of knowledge about waxbills that may be different than other finches!
Congratulations on deciding to adopt them but keep in mind that you should have a good knowledge on them to care for them properly. Maybe the breeder can tell you if he/she does anything differently with waxbills than with regular finches