I bought the upgrade from LR 4 to LR 6. The installation went fine. I found out that I now have LR 4.4 (0.9 GB) and LR 6.8 (1.4 GB) on my computer. Can I deinstall LR 4.4 without corrupting LR 6.8?
Yes it is OK to remove older versions without problem.
... and, to free up some disk space, it is also OK to remove old catalogs which are not removed when you remove the software, once you are sure the copied and upgraded catalog works as supposed with the new version.