Me and my ex-husband have been divorced for 4yrs,we reside in Pennsylvania.When he retires at whatever age he decides to retire, am I intitled to half of his pension? we have a divorce decree, but im not positive it relinquishes my rights to his pension.

If you did not have a Property Settlement Agreement and a special order which gives you part of his pension prior to the divorce being entered, the answer is likely no.  It would seem you waived your rights to his pension if you did not protect this right beforehand.  If you had a lawyer representing you at the time, you should consult with them on this issue.