The Sandisk Ultra 4 GB SDHC card is in the Ultra series for extra speed but not as fast as the Sandisk Extreme series. The Ultra series is class 4 or 15 MegaBytes per second for reads and 10 MB per second for writes.
Standard SD cards are available up to 4 GB. SDHC covers 4 GB to 32 GB then SDXC covers 32 GB up to many times the largest size we can manufacture with the most advanced theoretical technology. All camera and computer manufacturers should immediately jump to SDXC to handle the 64 GB cards due soon.
The regular economy Sandisk cards are class 2, fast enough for most cameras and ok for copying a GB of images form a card to a computer. The Ultra series are twice as fast, at class 4, and faster than most cameras. The extra speed makes no difference when using most cameras. The extra speed does make a difference when copying images from the card to a computer. The extra read speed is really useful when copying the contents of 8 GB or 16 GB cards to disk. The Extreme series is twice as fast as the Ultra series. I recommend the faster Extreme series when you use larger cards, 16 GB or 32 GB, and have to copy them while travelling. You have less time when travelling. Something as simple as electricity can be limited when on the move. The extra speed of the Extreme series is worth the extra dollars.
The Sandisk Ultra series is the best choice for holidays where you return to a hotel at night. The Extreme series is the type of card you invest in when a hotel room is a once per week luxury in between hiking.