Don't worry. You will not loose your archive of images, and you will not loose your edits.
After installation of Lr4, when you open it for the first time, Lr4 will ask you if you want to update your Lr3 catalog. Say <Yes>. Lr4 will create a copy of your Lr3 catalog and update the copy.
Lr4 will not overwrite your Lr3. You will still have Lr3 on your computer, and you still can open the existing Lr3 catalog with Lr3. But Lr3 cannot open the updated Lr4 catalog.
So once Lr4 is installed, don't do any more work in Lr3. Use Lr3 only to check things out. Or else, you will create two different ever diverging catalogs.
Lr4 is quite a bit different from Lr3. So it's good to leave Lr3 on your computer until you are familiar with Lr4.
