Jordan Kemlo, 18, admitted carrying out the Peterhead sexual assaults between 2011 and 2014.
At the High Court in Edinburgh, Lord Burns ordered Kemlo to be supervised for five years following his release from custody.
The judge said there was no alternative to custody.
Defence advocate Drew McKenzie told the court: "He did express that he is remorseful and that what he did to this boy was wrong."
A teenager who "never thought about the consequences" of repeatedly abusing a young boy has been jailed for four years.