As far as I know, Lightroom 4 will run on Windows 10. But if it is necessary for you to upgrade you will have to pay full price. You would be able to upgrade for the upgrade price in your country.
Do i have to pay full price for 6 if i do have to update?
You don't have to pay the Full price.You can upgrade to Lightroom 6 from $79(in US) Refer my answer to this thread How How to update to Lightroom 6