You will be able to upgrade to 3.5 but not to 4.1
VMware KB: Upgrading ESX/ESXi 3.5 or 4.0 to ESX/ESXi 4.1
Best option would be to upgrade using a CD/ISO but you can only upgrade to classic i.e ESX 4.1 not ESXi 4.1
Also ensure your hardware is on the HCL for the 4.x version
