if he has to invite all of his friends and servants can be repeated for sending multiple invitations then it will be $4^{6}$
as first friend (F1) can be visited by any of 4 servants. =. 4 ways
as second friend (F2) can be visited by any of 4 servants. =. 4 ways
.
.
as sixth friend (F6) can be visited by any of 4 servants. =. 4 ways
so total number of ways are : $4^{6}$