Yes it is possible to buy a stand alone version of LR as an Upgrade. But you don't really go from LR 4 to LR 6. Once you install LR 6 LR will still be on your computer and work with it's original catalog file. LR 6 will make a copy of your LR 4 catalog and convert that copy to a LR 6 catalog.
To get the stand alone version you need to go to the Product page of the Adobe website and click on the link to BUY LR 6 then select the Upgrade instead of the Full version. If you are having problems finding the Product page try a Google search for it. Actually it is on the bottom on almost every Adobe website page in smaller type.
Another problem, my Olympus O-MD E-M1 produces ORF files that are not recognized by LR4
First supported in Lightroom 5.3. Will never be supported with Lightroom 4.
No issues opening these files with Lightroom 6.
Or use the free Adobe DNG converter to convert O-MD E-M1 Raw files to DNG format then edit the DNGs in LR4.
Thanks all for your answers !
I have found the update (75 € instead of 130 € for the full version) and it works perfectly