I want to pick up a used frame set and was wondering is there a difference between these two frames? or should I spend a little more and go with the caad7. I am not too familiar with the cannondale frames any help would be great!!

The biggest difference between the CAAD4 and CAAD5 was alittle bit bigger seat tube and down tube where it meets the bottom bracket. This made the frames a little stiffer and lighter.
The CAAD6 attempted to use an integrated Cannondale specific bottom bracket that never really caught on.
The CAAD7 and CAAD8 front triangle are pretty much the same. The biggest difference between the earlier frames is the Optimo shaped down tube and top tube. They have an oval/triangular cross section and the tubing is a little thinner.
The CAAD8 seatstays still have the "S" curve, but the sides are flattened out more, too improve the ride quality.
For the 6/13, they took a CAAD8 frame and chopped out the top, down, and seat tube and replaced them with carbon.
If you watch Ebay, there are still new CAAD5 framesets/bikes showing up as stores clear out there stock. Ask your local shop, they may have a new one stashed away they will let you have at a good price. The shop here even has a CAAD3 hanging on the wall still!

I had gotten this from a Cannondale Rep when I inquired about the differences between the CAAD4 and CAAD5 frames:

Road CAAD 4 55 cm frame 1270 grams
Road CAAD 5 56 cm frame 1270 grams

So its a little lighter when comparing apples to apples.

I went with the CAAD4 when I recently bought a bike because it was an '04 model and I got a considerably better deal. I'm also more into it for recreation and not to race.

I do believe the CAAD4 is all but gone from the Cannondale lineup at this point.

Another big difference between CAAD 4 and CAAD 5 is that the 5 has an integrated headset. I would go with the 5 because I loved the one I had until I moved up to a 2005 R1000 (CAAD 8). The CAAD 5 frame is a bit heavier than the CAAD 8 but ride quality is very close between the 2.

Either way you go you are definately buying a high quality frame.