There are several options for that.

I think that many MT4 brokers offer PAMM or LAMM options in order to manage several accounts. I am not able to recommend any of them specifically.

An other option, if MT4 is not mandatory for you, is to do it with Oanda using their FxManager application where you can define different fee schemas.

In all cases the investor and the manager have to sign a power of attorney in order for the manager to get the access rights to the investor account.