-Lightroom serial number will work on both Windows and Mac versions https://forums.adobe.com/thread/2179765
Lightroom 4 cannot be updated to Lightroom 6 via a free update. You can purchase an upgrade to Lightroom 6, and the link has been provided in the previous response.
it means that if i don't want invest in lightroom 6, because I already buy lightroom 4 i stack with lightroom 4 forever?
Can i return somehow lightroom 4 and after that buy lightroom 6?
Do you know how it works with photoshop? if I have CS6 version can it be upgraded to CC6 version? or do i need to buy it as well?
How long have you had LR 4?
Have you ever purchased software before? Once you open the package it came in or installed it you can't normally return it (there may be exceptions to this rule about software).