As a DM, I would let the 4th level player gain the services of a squire as a paid hireling (which has no Leadership requirement). The "paid" part can be disguised as "upkeep" if you wish, but the important thing is that the squire is initially in it mostly for the money or glory.
Over time, the squire develop a true loyalty and become a follower or cohort. This should coincide with the PC picking Leadership as a feat. If the PC shows no interest in taking Leadership ("Gee, I already have a boon companion for free, so why should I waste a feat?"), then the squire never really develops loyalty and may ultimately betray the PC (or be reverse-engineered so that he already did so)! If the PC goes hot about this, shrug and say, "You had plenty of chances to take Leadership and cement his loyalty, but you got greedy and chose another feat..."
As for class, I'd say Fighter. NPC classes are for non-adventurers, and this squire is going adventuring. If he's a "Squire James" sort of character, then start him out as a Rogue that plans on advancing as Fighter (perhaps not following the plan immediately...).