I know this problem, as I've seen it myself. My regular season is 21 weeks, and my playoffs are a combined four. I have Two Rounds of playoffs, each of which are two weeks (obviously meaning four teams make my playoffs). Unfortunately, you cannot make one playoff series one week while making another playoff series two. It would be very ideal though, as then I could have six teams in the playoffs and give the top seeds first round byes.
As I said above, my playoffs are two rounds at two weeks each. I think it's best to have two week playoffs, just in case a team has an off week, or a week that was just off the charts.