Update: I have decided against MDF. One of the chain home stores improvement stores has edge-laminated boards of the width I need. While they are pine, I believe they offer better strength. The is higher, but that is fine. I will still be banding the shelves in aluminum channel on all sides except the back. The backs will be mounted into channel that is attached directly to the wall. Sagging should not be an issue across the 48-inch span.
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"Its not a matter of being ready as it is being prepared" -- B. E. J. Taylor