(moved from 4-4 point 3-3 invasion)
This is a variation I have been seeing lately. Sometimes instead of 4 I see b. This is exceptionally frustrating since it usually happens when I want to play c. ( Usually because I have some stones nullifying the ponnuki, and am able to make great use of the thickness. I could respond with a but it seems like I've given up more than I had to.
White is thoroughly surrounded. threatens to kill. Later, - threaten seki. (The last sequence is ambiguous, since is worth 3 points by miai counting, the same size as Black's hane-tsugi.) White's corner is worth only 6 points.
Compare this result with the typical joseki: