We need a 4 user licence, can we buy two 2-user licenses?
No this is not possible. You cannot combine two 2-user licenses to compose a 4 user license. The number of computers corresponding to the license (=2) is encrypted into the license key and checked on startup against the actual amount of OsaSynced computers. So two 2-user licenses can only be used if not all 4 computers sync together but only if they sync in pairs.