Your images are not "In" Lightroom. They are wherever you placed them on your hard drive or drives. So no you won't lose them.
You need to install LR 6 first and it will find your LR 4 catalog file, Copy it and convert that copy to a LR 6 catalog.
Then once you open LR 6 and all your images are still there, which they will be, you can uninstall LR 4.
Great! Thanks for the QUICK answer. I have been using LR6 for about a month now, and saw my old photos there, but didn't know how the removal of the program would affect it. Thanks for confirming for me.