For me, a lot would depend on whether there were decent backup options in the next few hours. If these are the last or only flights of the day, then I would be hesitant. Going, it seems worst case is getting a one-way rental and driving 4 hours. Consider reserving a car, if you don't need it you only out the time you spent reserving it. Coming home, it comes down to the other flight options. If there is a later DEN-PHL, I would book that, and maybe aim for standby on the earlier if you make it in time.