I agree with adgal, don't make a big issue of it. It probably wasn't your place to talk to the other boys about it. I would have told their parents and had them deal with it as they see fit. I wouldn't like it if someone else spoke to my DD when she was 4 about sexual things. I want to do that myself.
Trampolines are a safe toy when used correctly. Make strict rules about what the kids can and cannot do on the trampoline. Never leave them unsupervised on it until they are older.
I would ask your daughter if she has seen that somewhere to start.
I would ask her why she did that and how she felt about it.
Sometimes kids just dont understand what they are doing. She is 4 as well and doesnt understand sexual feelings.
I do agree with the supervision on the trampoline.
As for the "moving around in a sexual way". Masturbation in kids this age is super normal. It feels good to them even if they don't understand it.
I wouldn't make a huge big issue out of it. Explain to her that her private parts are for her to touch only, and that it always needs to be in private. Keep it very calm as you don't want her to feel badly about it. I'm sure others will chime in with more information for you, but that is basically how I would handle it.
You are right your husband isnt ...does he think it will stop ..tell them no and supervise , children get hurt badly on trampolines they need supervision , a 4 year old will copy what she is seeing .Ask at you local ER how many injured children are brought in ,its mind boggling ..