The three wire config is two lives plus a neutral.
There is 120 volts between each live conductor and the neutral and 240 volts between the two lives.
The four-wire config is essentially the same as far as the voltages across the lives and neutrals.
However there is a ground conductor (just like the round pin on the power cord on your computer or printer) that grounds the metal case of your dryer to protect you from a potential electric shock.
I imagine this green conductor is what's called the euquipment ground conductor, right?
PS I don't like that bottom picture. Where's the strain relief? A bit of rubbing from the sharp edge of that hole and you can have the conductors touch each other or the case itself. Hope it was only removed for demonstration purposes and this is not how the actual dryer is used.
(I have a 110-volt 15-amp electric dryer at home. It has a strain relief at the hole where the cord enters the cabinet