The model 5 is available in 4", 5", 6", 7" and 8". The model #8-4 is only available in a 4". The 8-4 has a wider bevel than the 5-4 and has a sharp top edge which some like and some don't. For a carry knife I like the 8-4 better if you are looking for a small knife. If you want something bigger you will want to go with the #5 in whatever size fits you.
Dwight