You will need to have unity android plugin (you need to buy it in the store)
I guess you had the free Android basic license for Unity 3.5, however each major version needs it’s own license. Licenses are backward compatible, so you can use a Unity 3 license for Unity 2 and 1 but not for Unity 4.
If you have a license for Unity 3 you can buy an upgrade to Unity4. Just take a look at the store