Converting 4 wire supply to 3 wire outlet
No, you absolutely should NOT connect the grounded (neutral) conductor to the grounding (earth) conductor. 

If you have two ungrounded ( hot ) conductors, and you only want to use one. You can simply cap the unused conductor using a twist-on wire connector, or similar device. 

However, when two ungrounded conductors are run, there's almost always a reason. It could be a simple multi-wire branch circuit, where one of the conductors is meant to feed other devices. Or it could be a multi-wire branch circuit, where one of the ungrounded conductors is switched. The latter is common where receptacles are split to allow half to always be hot, while the the other half is switched.