Whoa! How did we jump from a rude comment to school bully?
Honestly, this isn't something I think requires intervention. If you had been present, it would have been a good teaching moment and appropriate to tell the little boy it wasn't nice and to show him how to include other children. However, you weren't, so the child you should be concerned about is your own.
Tell your daughter you're sorry her feelings were hurt, and validate for her that the little boy wasn't very nice. Then, turn this into a teaching moment for HER. Help her to understand that sometimes people say or do things that aren't nice. It makes us sad, but we should still treat them nicely, and try to be their friend and a good example.
Also, teach her some things to say in response the next time someone (anyone, not just the little boy) someone is rude or hurtful. It is our job as parents to teach our children to become independent, which includes knowing how to deal with less than nice people. Starting now with relatively innocuous situations will give your daughter the confidence to handle herself and not be crushed by normal childhood conflict. Running to her defense and demanding apologies left and right teaches her she can't handle things on her own.
And how do you know neither of your children hasn't sometimes been "a snotty little kid who says mean things?" I can't tell you the number of times I've had to intervene during playdates because someone else's child has been rude or hurtful to mine, and not said anything to the parent. I also know that my kids have been the ones to dish it out at times. No child is perfect.
I must also offer some child development info here. Age should matter. A 4-year-old just doesn't have the same intent of action as an older child. The little boy didn't want to play with your kids anymore. Older kids know to suck it up or at least be gracious about turning down a request to play. A 4-year-old is still learning. He was not being cruel. You, the adult, however, are being cruel in your assessment of this 4-year-old.
Now, if there's name-calling, taunting, physical aggression, etc, by all means, approach the other parent. That is cruelty that needs adult intervention.
For this though, your husband should really apologize for his behavior, say he overreacted, and offer a conciliatory plate of cookies. Why turn a little childhood tiff into a fight between adults?