there's no direct upgrade available from ESXi 4.x to ESXi 6.0, you will have to go from 4.x to 5.x and then towards version 6.0 upgrade.
You can upgrade an ESXi 5.0.x, ESXi 5.1.x, ESXi 5.5.0, or ESXi 5.5.х host directly to 6.0.
The details and level of support for an upgrade to ESXi 6.0 depend on the host to be upgraded and the upgrade method that you use. Verify support for the upgrade path from your current version of ESXi to the version to which you are upgrading. See VMware Product Interoperability Matrixes at http://www.vmware.com/resources/compatibility/sim/interop_matrix.php.
There's a detail documentation on following link, from where above quote has been taken (Lesson 8). This will give you systematic approach towards upgrade path.