To find out which Update you have installed you can check the build number on the update site at http://www.vmware.com/patch/download/
I put together the ESXi 4.0 patches and updates for a better overview.
GA-Version: VMware-VMvisor-Installer-4.0.0-164009.x86_64.iso
ESXi-4.0.0-patch01 - vmware-esx-firmware-4.0.0-0.5.175625.i386.vib
ESXi-4.0.0-patch01 - vmware-esx-tools-light-4.0.0-0.5.175625.i386.vib
ESXi-4.0.0-EP1 - vmware-esx-firmware-4.0.0-0.6.181792.i386.vib
ESXi-4.0.0-patch02 - vmware-esx-firmware-4.0.0-0.7.193498.i386.vib
ESXi-4.0.0-patch02 - vmware-esx-tools-light-4.0.0-0.7.193498.i386.vib
*ESXi-4.0.0-update01* - vmware-esx-firmware-4.0.0-1.9.208167.i386.vib
*ESXi-4.0.0-update01* - vmware-esx-tools-light-4.0.0-1.9.208167.i386.vib
*ESXi-4.0.0-update01* - vmware-esx-viclient-4.0.0-1.9.208167.i386.vib
ESXi-4.0.0-patch03 - vmware-esx-firmware-4.0.0-1.10.219382.i386.vib
ESXi-4.0.0-patch03 - vmware-esx-tools-light-4.0.0-1.10.219382.i386.vib
ESXi-4.0.0-patch04 - vmware-esx-firmware-4.0.0-1.11.236512.i386.vib
ESXi-4.0.0-patch04 - vmware-esx-tools-light-4.0.0-1.11.236512.i386.vib
ESXi-4.0.0-patch05 - vmware-esx-firmware-4.0.0-1.12.244038.i386.vib
ESXi-4.0.0-P06 - vmware-esx-firmware-4.0.0-1.13.256968.i386.vib
ESXi-4.0.0-P06 - vmware-esx-tools-light-4.0.0-1.13.256968.i386.vib
*ESXi-4.0.0-update02* - vmware-esx-firmware-4.0.0-2.17.261974.i386.vib
*ESXi-4.0.0-update02* - vmware-esx-tools-light-4.0.0-2.17.261974.i386.vib
*ESXi-4.0.0-update02* - vmware-esx-viclient-4.0.0-2.17.261974.i386.vib
ESXi-4.0.0-patch07 - vmware-esx-firmware-4.0.0-2.18.294855.i386.vib
ESXi-4.0.0-patch07 - vmware-esx-tools-light-4.0.0-2.18.294855.i386.vib
ESXi-4.0.0-patch08 - vmware-esx-firmware-4.0.0-2.19.332073.i386.vib
ESXi-4.0.0-patch08 - vmware-esx-tools-light-4.0.0-2.19.332073.i386.vib
ESXi-4.0.0-patch09 - vmware-esx-firmware-4.0.0-2.20.360236.i386.vib
ESXi-4.0.0-patch09 - vmware-esx-viclient-4.0.0-2.20.360236.i386.vib
André