The arrival & departer difference is 76 hrs at Beijing can I avail 72 hr free visa & 4hr in Airport?

Actually, the 72 hr starts from the 12:00am on the next day, so it is ok for you to avail 72 hour free transit. But you should better confirm this with your carrier airline if they allow to board plane in this situation. Anyway, 76 hour transit can not be divided into 72 +4 hours.