Where can I convert old license key version 3 to 4? I prefer the conversion on the web.

You mean key from RouterOS v3 to RouterOS v4? You don't need a new key, just upload packages and reboot. Upgrade will happen automatically.

If you mean from XXXX-XXX SoftID Format to XXXX-XXXX then click on the 'update key' button in the winbox license menu. That's the only way