A class 4 card will work OK for still pictures but it is not fast enough for HD video.
The class 4 card will drop frames when recording which will make the video look jumpy when you play it back.
I use the San Disk Ultra for HD video.
I just bought the new Sony HX9V and am psyched about it, but will admit that I really don't know much about memory cards. There's standard, ultra, extreme, etc. And lots of different speeds and classes. The Specs for the camera say to get Class 4 or higher. Will any old Class 4 card work without detracting from the quality of the 1080/60p video the camera can take.