Other people have gone back to 4.18 without problems. Sounds like LW 5's preferences are the problem for you. However if you delete the entire LW preferences folder you will lose your download history. You will need to make a backup copy of a file called downloads.bak from the preferences folder. Then return that file to the preferences folder after installing LW 4.18 whilst LW is closed. I'm not sure of any other files needed for back up from LW 5 since it introduced a new manner of using the preferences folder.

You meant LW versions 4.16 & 4.18 right? Not 4.6 & 4.8 which are really quite old. Get LW 4.18 from here Lime Wire » Download Out-Of-Date Installers. If you were trying to install 4.6 & 4.8 then that might have been the problem.