Most people find that score increases come in "jumps" - it's not really linear. Further, a lot of people don't see much improvement at all at the 4-week mark because they haven't learned enough yet, and they're still practicing / getting used to what they have learned so far.
You did improve by 40 points, and you're only at week 4 of the 9 week program. I think you're in good shape. You don't mention a specific test date, so I'll assume you have some flexibility there - just keep doing your work, assessing your progress periodically, and making adjustments as necessary.
Also, when you take MGMAT CATs, you can use this article to help analyze your results:
http://www.beatthegmat.com/mba/2011/02/ ... sts-part-1