Taken from a Bassplayer article:
On his 4-string electrics, Les uses Dean Markley nickel roundwounds--but here's the secret: his set consists of two A strings (.080) and two G strings (.040). "I've always loved the sound Tony Levin gets on the [Chapman] Stick with light-gauge strings. I wanted to get that same punch, so I started to use a lighter E string, and then I switched to two G's to make my chords sound better." He strings his Carl Thompson 6-string with a standard light-gauge set.
http://www.bassplayer.com/artists/les93.shtml
that article is a good read
Note: do not be confused into thinking that Les tunes his 4 string basses AAGG.. he still tunes EADG.. it's just that the string he tunes to E was meant to be an A string, and the string he tunes to D was meant to be a G
it's a pretty common misconception to think that les uses some weird tuning.. but he only uses weird strings
