As far as I know, there would be nothing that would prevent you from performing such an upgrade. Your challenge will be to find an upgrade to Lightroom 5 to purchase. Adobe doesn't sell it anymore. So you will have to find it from another reseller such as Amazon or B & H Photo, etc.
Ffantastic, thank you very much for the answer !
The solution is: I already have the english version and want to sell it (in germany) - the potential buyer is asking, because he has the german version of LR4 !
Is anybody here in the forum who knows exactly, that it definetly works ?