Yes, I have used an MH in the past, on a particular experienced horse who was also raising his head and neck too much, and it was in a lesson. Outside of that I have never used a MH - it is far better (in the short term and in the long term) to resort to proper training, rather than a gadget to solve a problem.
Having not seen you or your horse, its hard to determine what would help you best. At 4 years old, your horse may simply not have the strength to carry himself with his neck down, and it would be foolish to resort to a gadget in that case. In general, with many 4 yr olds (of course it depends on the 4 yr old in question), I would be concentrating on getting forward, and not worrying about where the head and neck is at this stage. If your instructor's intent is to teach the young horse where the head and neck should be, then yes, I have seen draw reins (which have a similar action to a MH) used short-term for that, by experienced riders who have brought on young horses before, and who know what they are doing.
A good rule of thumb with horses is, if you don't know what the piece of tack does, don't use it, unless you are under the instruction of someone more experienced than you, who you trust to teach you how to use it wisely. Are you going to have lessons with the MH on? Or has she/he suggested this piece of tack and left it at that? Realistically, I know you don't want a flaming, but it is hard to answer your post without more detail - up to you.