As a professional musican and a Music Teacher but not a brass player, may I offer some advice. Playing a proper trumpet requires the teeth of the child to be fully formed so generally children would not start until 9 at least. There may be a band in your area which might be a good entry point to musical activity. At Four years of age the emphasis should be on seeing music as fun in a games context. If you live near a good music school, they may have a Pre Instrumental course which could be a stepping stone to learning a musical instrument.