Sounds a lot like a failure in the idiotically designed speed control module.
If 4 slowed down whilst 1-3 were failing I would lean towards a wiring failure. If it stayed at full power then I would lean towards the speed control module.
Most speed control modules have a power input, a power output and a control input. If you bridge the power input and output wires together on the blower module with a bit of wire, does the fan come on? If so you can use a switch wired to the pins on the plug with some crimp spade terminals as an emergency bodge until you can get a new unit. On the connector: Power input is normally at 0V, power output should be at 12V when the control unit is disconnected and the control input is somewhere in between, changing with the temperature knob.
The units typically change the resistance of the ground connection of the fan, hence the counter-intuitive voltages. This is a stupid way of doing speed control designed to bake the transistor to death. Why they don’t PWM drive it is beyond me.
If it is a wiring fault, don’t assume that it’s in the 12V feed – there may be more connectors (typical failure points) in the 0V feed. Measure the resistance from the motor’s connector to 0V at the control unit plug and to the fuse to determine the failed path.
