If you use an LM339 comparator with a pot giving a 4.5 volt reference to the - pin, it will turn on when the input on the + pin rises to 4.5 volts. This is open collector out put so you need a pull up resistor. If you swap the inputs, you get inverted output that you can use to ground the LED at 4.5 volts so no pull up is required.
There are 4 comparators on that chip. Ground all unused inputs. With 4 different reference voltages you could use it to make a bar graph of 1 input.