The cook time and temperature I need is not an option on my crockpot How do you adjust the cook time for a recipe that says cook on low for 6 hours to coordinate with a programmable crock pot that only has settings for 4-6 hours on High and 8-10 hours on low?
Go for 8 hours on Low.

2 more hours on low shouldn't hurt anything too much, for many slow-cooking extra cook-time can actually be beneficial. More cook-time usually means a more tender finished product. 

Whatcha cookin? That would help us answer this question better.