According to Showtime's website, they just began production on the 5th season this summer... so my guess is January 2008, unless they're like the Sopranos (taking 1-2 years to finish production!). I've never watched the show, but keep meaning to rent the DVDs. I was a huge fan of Q-eer as Folk & Sex and the City, so I figure L Word would appeal to me.
Oh, and sorry about the censoring above, but they wouldn't let me write the show's title! I assume you know what it is...