apt1002: 4-2 is a good answer to 3-3.
There are various ways to enclose the corner more securely: was an idea of Kubouchi Shuchi. Black a is common but Black b is probably most popular from the point of view of corner safety.
This isn't often seen in pro games. White a, Black b is a continuation seen in a couple of examples.
erikpan: Why not white b, or one to the left of b? This would seem better from the point of view of miai moves. xela: Depending on the whole-board position, those moves might be perfectly OK too. Most of the time though, I think white would want to prevent black making a second shimari. (See Allowing two enclosures.)