Are the 400 users at the same time?
If you use 400 UDP users, even if they don't want the stream they receive it. So you're downloading the banwith. But your bandwith is constant use.
If 400 HTTP joins, you don't download the bandwith of your clients id they don't want to see the stream. Your bandwith will also be variable in function of how many clients are getting the stream. I think also this will produce more CPU usage, than UDP option.
I'm not guru, just net knowledge. But i will use HTTP join.
By the way see if you could use Multicast in UDP. This will depend on the routers to your clients and yours.