071226-2001 EST USA
A Stanley EFR5366X (DigiKey 404-1090-ND) in series with your 4.7 K resistor will give lots of brightness. If you use a 470 ohm it will be much brighter.
Or a less bright LED in series with the opto input.
In each case the LED is on when current flows into the coupler.
What is the load on the output? This will be a major factor on the output voltage. If it is a 2n7000 FET, then there is virtually no load. If it is a 100 ohm resistor to ground, then there won't be much output voltage. If it is 100 ohms to +5, then there is lots of output voltage, but the opto may not be able to pull it down. So it is true if the LED is in series with the 4.7 k resistor, then you might excessively reduce the output and caause a residual glow in the LED. You have to more completely define the circuit.