If you purchased a retail version of CS4 then the serial number was on a sheet of paper inside the box. If you purchased it online you were emailed the serial numbers.
If you no longer have records of the serial, you will need to contact Adobe Support.
Forget it. Suites can only be upgraded to suites. Trying to just upgreade a standalone AE with a suite serial will never work and contacting support won't do anything, either. Return the package, upgrade/ sidegrade your suite or buy a fresh AE license/ get Creative Cloud. So many options these days...