H4 STAMPING AFTER H1 EXTENSION WITH FUTURE DATE

Hi,

My current H1-b is valid till March 2019 and my extension has been approved with a start date from March 2019 to March 2022. However, my spouse will be moving to U.S.A in December. In the D.S 160, should we be add Old 797 receipt # or the new one which starts from March 2019 for H4 stamping?. If I use the old one, will the H4 visa be valid till 2022? Or If I use the new receipt #, can she enter U.S.A in December even if the start date is in March 2019?