Problem: if the profit margin on the 3GS is $100 and the profit margin on the 4G is $150, there are no damages. No damages, no case.
Newer phone is likely to be more expensive to produce than 3GS which used 3G tooling for the case.
Also remember the newer phone has higher res screen (more expensive), bigger battery (more expensive) front facing camera (additional cost over 3GS), better rear camera (more expensive).
Therefore the newer phone is likely to initially have a smaller profit margin than the 3GS.