You might need a new bolt head for proper head spacing. Each bolt head was stamped with a number '0', '1', '2', etc. and generally they easily screw off and on. You would need a set of No-Go gauges to make sure the bolt head fitted to the No4 bolt provided correct head spacing for your rifle. You might need to play with a few bolt heads until you get proper spacing.
The major difference between a No5 and No4 bolt is that early No5 bolt handles were drilled out in order to lighten the overall weight of the rifle.