its a myth that a mortar crew is only Three,
a commander,gunner,and assistant gunner,
your fourth's role is ammo carrier/ security,when things are about to get very boring his role is to pick up a weapon and defend the weapon system and core crew.
Actually Simon and Mick have given great value for your money as most companies don't do a fourth figure,and count the weapon system as a figure.
if you want get period accurate mortar crews including all roles were on upto an eight man section.So four figures gives you plenty of display option's when modeling more than one mortar.
Mark