you can indeed fit 4 wheels, even 19's in the zed.
i got 275 and 235 in there
take spare tyre and boot mats out, one REAR under the strut bar
one REAR over the strut bar sitting behind the headrests
two fronts in the front seat sitting the same way as they would fit on the car, so side on, next to each other. if you angle them slightly toward the door it helps them from tipping. also you can wind the window down reach in and wrap a bungee round them and the passenger door handle to stop them falling on you.
hope that helps.
EDIT: when youve done it a few times it only take seconds tp get them in