Everything S4 has, plus the sole
An S5 boot is fully waterproof and additionally has a penetration-resistant midsole and a profiled tread for grip on muddy and loose ground. It's precisely on demolition sites, in groundwork and in sewer work that the combination of water plus sharp debris occurs most often, and that's exactly what this class was made for.
Mind the fit
Boots are often bought a size too big so they're easy to put on and take off, but a boot that slides up and down causes blisters and instability. Choose the size based on your work sock and look at models with a calf adjustment. If you work in the cold, there are lined versions available; also check out thermal socks.