can I have a default judgement overturned 4 years after the default occurred without my knowledge?

I am not familiar with Nevada law, but if it is the same as the states in which I practice the answer is yes, if you were never served with process.  If you were never properly served, the  Court never had jurisdiction over you and any judgment is a nullity.  If you were served, however, even if you never got notice (for example, if the process server left the complaint with your wife at your home, and she forgot to give it to you) you are probably too late.