Why would you want to use ESXi v4? It is very old.
From the VMware website, you can download ESXi 5.0 or higher. Browse to www.vmware.com. Click on DOWNLOADS on the left pane. Click on vSphere.
You can use any ESXi version for free with limited capability without a license key. For example, you cannot use vCenter Server to manage a free ESXi server. You can also get a free 60-day trial license with full functionality.