1) Buy this book Mastering VMware vSphere 4
2) Read this document vSphere Admin Guide
3) Read the vSphere Maximums PDF
4) Read the SAN Setup Guide
5) Read the VMWare forums.
6) Read every PDF from vmware.com about vSphere you can find.
7) Do installs, do upgrades (every method), Setup and try all the new features. You will find vSphere 4 to be a much bigger upgrade to 3.5 than you'd expect.
If you do vSphere all day long, you will fail this test (I do and I did the first time). You must read the forums and get involved in the VMWare community. They will throw some insane questions at you that any decent admin would never encounter, the ONLY way to know the answers to these is to read the forums for obscure issues bad admins have run into by setting up their ESX servers and vCenters incorrectly.
Study hard and good luck.