Maybe the reason CBS isn't promoting season 4 is that they feel all the promotion and advertising for the previous seasons was enough to make people aware that season 4 will be out soon. Or they have found advertising isn't increasing their sales sufficiently beyond what they would expect from the hardcore fans?
There really isn't any big standout episodes from that S4, really ("Family" is the closest one I could think of). With S3, CBS promoted the heck out of BOBW.
That was my thinking as well. That holds true for the rest of the series until you get to "All Good Things" of course.
All the other 2-parters are either pretty so/so or require you to be a pretty involved fan to watch. BOBW is so widely known and so well liked it was sure thing for CBS and Fathom.