Hello: A single pole, double throw switch would be used on the upper element to switch power from upper to lower once the upper was hot. That’s essentially what US heaters use. We refer to power as line (power in) and load (power out). I’m guessing that line is “R” (1&2) and load is “S” (3&4). Note that on “S”, when one contact is “made”, conducting power, the other contact is “broken, not conducting power. This would be where the upper and lower elements would take power.
As nobody likes unwanted sparks, you might want to have somebody good with electricity keeping an eye on things while you turned the power back on 😎
There are others who post here who are probably clearer in their understanding of this thermostat and can enlighten us both!
ps. Possibly the manufacturer of this thermostat can tell you how they intent the device be used in a heater.