the sdhc cards tend to have a little better speed/quality. Also, sandisk is reputable and makes good product. If your going to buy a new card, I would just splurge for a class 10 card from a reputable brand. not that much extra, and ensures you will never have problems with the cam stopping recording. Hope this helps.
All that said though, shoot with the one that works until it doesn't anymore. the issue with the class 4 card is that your files/video might get scrambled or messed up and you may lose something you really wanted.