Once you are sure that you are happy with LR 4 you can uninstall LR 3.6. If you have any printing or developing presets that you made you may need to manually copy them from the LR 3.6 folder to the LR 4 folder, so see that LR 4 has everything before you delete the presets.
Thanks. I also find that when I open some photos in the Develop Module, the Module with the LR3 sliders come up. If I delete LR3.6, will this option also stop? Or is it part of LR4 ?