There are 150 new features in vSphere 4

If you are having VI 3 entreprise with an suppor it’s  … a zero cost.

The upgrade is quite easy.

You are able to achieve 30% Greater Consolidation than in VI 3.

You obtain 50% Storage Savings with Thin provisionning.

With DPN you gain 20% of Power Savings. Distributed Power Management isn’t new as it was introduced over a year ago in VI3. However, up until vSphere 4, this feature was only experimentally supported. VMware supports DPM in vSphere 4 now.