I'm pretty sure the Chevy 4500 is already dead, or at least on hiatus. They announced last spring they were allocating more production to the Colorado (which is made on the same line). Navistar was supposed to take over the Express cutaway in 2018, but got caught up in the Silverado MDT production. All of the large RV makers have removed the Express 4500 from their new floorplans, and I'm not seeing any new bare chassis for sale, only built-out cube trucks.
The new E-series has many upgrades (a few examples: lane keep assist, automatic emergency braking, active cruise control, new instrument panel architecture) that indicate they indent to keep it around a while.
The Sprinter being put into heavier service IMO is a non-starter, at least until they come out with a more powerful powertrain.