I had seen the ads for these and took a look at one last week, just to gawk. I can't see the need for it and I think it's a needless feature that adds weight, complexity and maintenance to the ordinary self-propelled mower. 'Personal Pace' and 'Variable Speed', Toro's two drive systems for walk mowers of 22" cut and lower are the simplest, most durable and efficient drives imaginable. When you have a rear-wheel drive mower, you have the best type of mower propulsion available. Adding near-useless front-wheel drive to a RWD mower accomplishes nothing good.
These are not automobiles, where FWD, AWD and 4WD are all better than RWD and have many applications - the concept just does not translate to a lawn mower. Ask yourself why FWD mowers spin their wheels so much that the tires wear out prematurely - the answer is because weight transfer on a mower with no suspension is always to the rear. Plus, when bagging, the rear gets heavier and heavier and this lightens the front end more and more.
Okay, enough, but if anyone has any practical application for 4WD on a walk mower, I'd love to hear about it.