try FX Primus. I do not have 400:1. but I saw it as an option when choosing my leverage.the broker is very good. never had any problems and when I lost trade due to platform issues they returned the money.
I doubt you will find any reputable places that accept USA clients that offer 400:1 its illegal in regulated countries so you are left with dodgy unregulated places so why take the rest.....
DEMO doesn't count - go live or go home - ?a_aid=112256