Did some digging.
1. Searched for any LP file leftovers after the uninstall. There were 1131 of them, even though I used a deep cleaner after the LP uninstall. I managed to delete most of them except two objects:
lastpass - passwords made easy
which are referred to as History items and I can't delete them.
Searched for registry LP leftovers and found a lot (exported 11 of them to .reg files). Deleted all, did a registry cleanup and rebooted. LP 4 installed correctly.
I have no idea what effect the leftovers have, if any, on the install problem, but the number of items you store on the computer and in the registry and the fact that the LP uninstall leave them behind is are unacceptable and are sure to cause problems.
Pls forward this to management and have them explain what they're gonna do about it asap.