Different major versions of Lightroom have different serial numbers. A serial number for Lightroom 2 will not work for Lightroom 4. You would have to purchase Lightroom 4 as an upgrade. Then you would need both the Lightroom 2 serial number as well as the new Lightroom 4 serial number. The reason for this is that an upgrade is sold at a reduced price, so The serial numbers from both versions is needed. However, Adobe no longer sells Lightroom 4. It is unsupported software, it will not ever be updated. A better option would be for you to purchase Lightroom 5. You might do some searching on different online retailers and find Lightroom 4. But Lightroom 5 would be a much better choice.