A 2 stage/multi stage heat pump just slows the indoor blower when the heat pump is running in first stage. So with a 4 ton 2 stage heat pump. When its in first stage you basically have a 3 ton running on a 4 ton duct system. And still have a good temp rise.
Get the air flow correct, and then find out why it has a poor temp delta. Could easily be that it was over charged if they had the blower speed/CFM set too high.