The LR4 serial is relatively easy to locate on your PC if you can't get it online. You'll just need to look at with with a text viewer.
On a Mac it's here:
/Users/[user name]/Library/Application Support/Adobe/Lightroom/Lightroom 4.0 Registration
On Windows it's here:
C:\ProgramData\Adobe\Lightroom\Lightroom 4.0 Registration.lrreg
Alternatively if you bought it from adobe try logging into the adobe page www.adobe.com , select manage account, select plans and products>view products.
That should show both your LR4 serial
Brilliant! All sorted, many thanks!