Can I delete LR4
Yes you can un-install Lr4. The program software is independent. Do NOT delete your catalogs (xxxxxx.LRCAT). The catalog contains ALL your data about your photos.
When you open a Lr4 catalog in Lr6 it will do a conversion update to the catalog (and leave a copy of the 'old' Lr4 catalog to archive)
How can I install LR6 using the upgrade on another computer? I installed it by an Adobe download....
If you have Lr6 installed- Just run it on the second computer- it will ask for the "Upgrade Serial Number" and then also ask for the "LR4 Serial Number". You need both Serial numbers.
You do not need to have Lr 4 installed. Only both Serial Numbers will be needed.
Download Lightroom- Buy Adobe Lightroom CC | Download digital photography software free trial
Convert to Standalone- How to convert a Lightroom CC trial to Lightroom 6
Update to v6.9- See links in "How Do I Update?" - https://www.lightroomforums.net/threads/whats-new-in-lightroom-cc-2015-9-6-9.30537/
Thanks very much for your clear info.