If a gun shoots 1.5 MOA then it's about 2" off target at 250 yards, not 4". It's a radius measurement when you’re talking about Point of Impact variance, and a Diameter measurement when you’re talking about the whole group.
Remember, all bullets are within 1.5" at 100 yards, which means in a 5 shot group two shots are .75" away from the group center, so three of them must be inside of that. Therefore, at 250 yards with a 1.5 MOA gun, worst case scenario is a round no further away than 2.25" from point of aim (that is if the shooter does their part).
Also, 4" groups are about the same size as a clay pigeon, if your gun can keep all shots inside of that at 250 you're good to go on just about anything you point it at! ~6mm