A Class 6 card has a higher minimum write speed than Class 4 (about 6MB/s vs 4MB/s), so in theory it can clear data faster. In practice on a Canon 500D, the difference may be small for normal still shooting, especially JPEGs. For occasional photos at low shutter speeds, card class is unlikely to matter much.

Where faster cards can help is during RAW bursts or video, since those create more data and can expose card-speed limits. However, several users report that on the 500D the camera’s own buffer/write performance is often the bigger bottleneck, and decent Class 4 cards can work fine for both photos and video.

The bigger factor is card quality and reliability, since not all cards of the same class perform equally well. A reputable Class 4 may be better than a poor-quality Class 6.

Bottom line: for your usage, a good Class 4 card will probably be fine, but a Class 6 gives a little more headroom for RAW bursts and video and is the safer choice if the price difference is small.