Result: 0/10 would not recommend. Even at 4 inches, it didn't add perceptibly to either the feeling of difficulty or exertion based on HR/pace. Further the recovery felt somewhat "sped up". Imagine running downhill with someone pushing gently on the small of your back

I will not bother with further experimentation (weight vest, juggling kettlebells, replacing handle with 45 lb olympic bar, etc).