1. I have iC6 and iC7 and CC3 on my computer. What content is lost when iC8 or CC4 is installed?
None
2. Assuming the desire of a user to do so, should the older versions of iC and CC3 be removed before or after the newer software is installed?
No fixed rule on this, but it isn't a bad idea to keep at least the last version installed for a while - just in case.
3. Will the programs either overwrite or store legacy content in their own folder, or integrate them with the new program folders?
No content is overwritten when a new version is installed, unless the user opts to have everything installed in the existing folders. During install the user can choose just where to have both default and custom content stored for each version.
4. Will there be an installation guide for customers who desire to retain their content and customer files but not the legacy software upon installing the new iC8 and CC4?
There has never been a guide covering this, but as stated in the answers above, no purchased or custom content from an existing installation is removed or modified if the folder structure is kept separate for each version.
As an example, I have iClone 5, 6, and 7 as well as CC 1, 2 and 3 on my system at the moment and each version has a separate set of content folders.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.