Can I safely create another 120V outlet using the fourth neutral wire from a 4 wire 240V line?
You didn't say what you're doing with the other side.  It depends an awful lot on that.  

If the circuit breaker in the main panel is 20 amps or less, and the existing load is 120V (i.e. between hot and neutral), you can make it a multi-wire branch circuit which is basically what you're talking about.  (By "multi-wire" they mean multi-hot.)

Why 120V only?  You can't do an MWBC if there are any 240V loads on the circuit.  

Why 20A or less?  Nothing about an MWBC requires that.  But a different rule requires outlets have the same rating as the breaker.  (Except 15A outlets are allowed on 20A circuits).  And disallows ordinary wired lighting on >20A breakers.  So if the breaker is 30A, you can't use common outlets or lighting.   

If you do MWBC, it's best to feed it with a 2-pole breaker, which, by definition, takes 2 full spaces in the panel -- never, never, never a duplex/tandem breaker that crams 2 breakers into one space. 

If the above rules don't allow you to do MWBC, then a sub-panel will handle it.