Apologies, to answer your question, yes, I was able to update from SL4 to SL6 without needing to enter my SL4 serial number, I presume the installer detected the already installed SL4. So just download the installer and run it, and enter your activation code (from Steinberg) when requested. In an earlier post, Robin clarified the following:
In my case, as I already have licenses on a USB-eLicenser, I was able to activate the license, run the installer and everything just worked.