The heater/air conditioner blower only works on high speed if I try to lower the fan speed setting it turns off.

Try all fan speed settings to determine if you have air flow on any (particularly High speed). I don't know the specifics of your Echo, but typically high speed is a straight-through connection for power to the blower motor.
If high speed works but none of the slower speeds do, the issue is very likely the blower motor resistor or the resistor connector.
If none of the speeds work, there might be a bad fuse for the blower motor or the motor itself might be bad.
If you can hear the blower on all speeds but still no air is coming out the vents, the issue might be either a damaged diverter door that is not properly directing air flow or possibly something got sucked in to the blower motor squirrel cage and is severely restricting air flow. -Rod

You are talking about your blower fan for the air conditioner and heater right? More information will be needed to solve the problem.

When you say it barely changes speeds, does that mean it doesnt speed up, or doesnt slow down? By that I mean, is it "stuck" on low, or high?

Most blower fans run at different speeds through the use of a multi-stage resistor built into the fan switch circuit. Low speed is a high value resistor, medium speed is a low value one, and high speed is the fan running without any resistance.

If your fan is running on high and doesnt seem to want to run on lower speeds, the resistor pack may be failing. If the fan is running on low speed only, it may be that the fan itself is beginning to fail, or that the lubricant in it has gotten gummy.

I would replace the blower motor resistor first then see if that corrects this. The resistor is located next to the blower motor under the dash.

You'll have to find the correct wiring diagram in order to trace down the fault in your system.. Once you obtain the wiring diagram you'll be able to see what else beside wire is between the #4 speed setting and the blower motor. My guess is you might find a high speed fan relay is triggered from the #4 fan switch setting, applying full battery voltage to the blower motor. If locating the appropriate diagram proves to be difficult you can try looking around the underhood and passenger compartment fuse panel to find a relay. Keep in mind not all relays are always in fuse boxes which makes the diagram important.

Check the wire connections at the blower motor and fan resistor, they are located under dash on passenger side. They sometimes have corrosion that burns the plastic connector. The resistor is responsible for all fan speeds below high. Hope this helps, let me know.

All cars are designed with a blower fan motor that pushes air through the ventilation system. This air is processed by a series of vents to direct the air to various positions. Example: heater, air conditioner and defrost modes. Most blower motors are power by a basic electrical circuit that utilizes a multi resistor or a solid-state resistor to control different fan speed modes (low, medium low, medium and high). If the resistor malfunctions it may lose one or more of the lower fan speed settings.
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Sounds like a bad blower resistor

Usually when the blower fan only operates on hi speed, its because of a failed blower motor resistor. You still have hi speed because on hi, the power going to the blower motor goes directly to it, as opposed to the lower speeds, when the power is routed through the resistor, thus lowering the voltage, which in turn lowers the speed of the fan. The resistor is usually located near the blower motor itself, under the hood back near the firewall.

The resistors are in circuit only on the lower fan speed settings.
On the highest speed setting, power is switched directly to the fan motor.
Check fan power supply fuses,
Check power is available to the fan when high speed selected.
If all above OK, apply power directly to the motor to verify fan motor runs- if it doesn't the motor may need replacement/repair.

You need to replace the blower speed control module, it is behind the glove box near the b;lower motor.