Winter aeration is good, but you'll want to avoid super-cooling. I doubt many, if any fish would get trapped in ice, but that 4' shelf is a great place for the winter placement of your diffuser to keep a large area ice free and avoid super-cooling. Summer diffuser placement should be in the deepest part.

Winter aeration is done to only keep an area ice free where oxygen can be dissolved into the water while unwanted gasses (Like CO2 and Hydrogen Sulfide) are released to the atmosphere, and to NOT circulate and "super-cool" deeper waters