tacky voltage indicator

          R1        V      D1         R3
Vcc o----/\/\/\-----+-----|>|-------/\/\/\----+
                    |                         |
                    \                         |
                    /  R2                     |
                    \                         |
                    |                         |
               GND  o-------------------------+

I1=I2+I3
V=Vcc-I1R1
V=I2R2
V=Vd+I3R3
Vd is the forward voltage drop in the diode, we'll assume 2V, that could be an orange or red LED.

After choosing a low value for R3 and low-ish values for the R1-R2 voltage divider in order to get V a bit above Vd when Vcc=5V, and tweaking it a bit, we can use for instance these values for the resistors:
R1=200ohm
R2=160ohm
R3=50ohm

When Vcc=5V  => V=2.08V, I2=13mA  I3=1.6mA
When Vcc=4.5 => V=2V,    I2=12.5mA I3=0mA
Basically, I3, the current going through the LED, decreases rapidly as we go from Vcc=5V to Vcc=4.5V

With Vd=2.1V (orange LED) and R3=20ohm it would be dimmer (I3=1.12mA) and it would also go out pretty soon, before Vcc hitting 4.7V
With Vd=1.8V (red LED) and R3=200ohm, it would be bright at Vcc=5V (I3=1.4mA) and it would be half as bright at 4.5V (I3=0.7mA), finally dying out at 4V or so.