Gardening in Raised Beds
No matter what size of garden we have, we gardeners always want more!
I would go for the two 8 foot beds. You can still rotate your crops as you would in 4 smaller beds, just by halving the area in each bed. Ease of access is key to successful raised bed gardening and it would soon become annoying working in a cramped space. Two 8 foot beds will give you the same amount of growing space but with better access, so it has to be the best option.
Also, when choosing your beds, try and make them as deep as possible as this will give you more options when choosing which crops to grow. For example, potatoes, carrots, parsnips and squashes will all grow better with a deep root run.
Good luck with your raised bed gardening. Please let me know if I can be of any further assistance.