If your Pro benefits are still current (eg if you bought 6 months worth of Pro, then you need to still be within that 6 month timeframe) then you can download 4.18.8 Pro from your personal download page. You'll see a link on the left side of the page 'Get LimeWire Classic'.

If your Pro benefits have expired, then you won't be able to download a 4.18.8 Pro.
If you choose to install a basic version & don't have Pro installers saved, or current Pro benefits, then you'll lose the Pro version originally installed