Yes. With Steel as his Mastery, Juggernaut can already use Metal Claw, Iron Defense, Flash Cannon, etc., every Steel-type move whatsoever that an Aggron is able to learn in any way. The four moves allowed are for those moves you want that are, in this case, not Steel-type, that an Aggron could learn.
Speaking of which, Aggron can have the Rock Head Ability (in the games at least), so recoil from Double-Edge, for example, would be absent.