Just a few examples of rotations covering 3 and 4 year cycles. Hoping to emphasise flexibility and opportunity, rather than strait-jackets - so have fun working out your own, favouring your preferred crops. For a longer cycle, look to Bob Flowerdew (on Gardeners' Question Time)
A "traditional" three year rotation (no potatoes)
A three year rotation (with a few new potatoes)
The four-year rotation I use on my plot
A four year rotation on a plot with white rot (making onions very difficult)
A four year rotation on a large plot, where you can let a bed lie "empty" every year