Is it possible to re enter in US after four year.?we have a valid green card.

Your parents will likely be found to have abandoned their residence. The fact that there green cards have not expired does not protect them from a finding of abandonment. Once an alien remains for more than a year, he/she is presumed to have abandoned his/her status. While this presumption can be rebutted, it will be difficult to do given they have been outside the country for four years unless they have proof they never intended to abandon their residence and that they remained outside for so long due to circumstances outside their control. They could seek an SB-1 visa. However, your parents will likely need to go through the immigration process again to obtain lawful permanent residence. You can read more about family immigration at http://myattorneyusa.com/family-immigration.