When I upgrade from LR4 to LR6 on my Macbook, do i need to uninstall LR for, or will the upgrade overwrite this?
LR6 will not overwrite LR4; they are both independent applications. I usually recommend installing LR6 and upgrading your catalog. You can the ensure LR6 is working OK. After a short time you can uninstall LR4. Actually there is no installer on Mac so simply send LR4 to the trash from Finder >> Applications.
You can buy from the following link. After clicking "Buy" change from Full to Upgrade; then select your existing LR4 product from the drop-down menu and the correct pricing will be displayed. Then simply choose “Add to Cart”
Thanks for the advice.