Using RF Link Transmitter/Receiver - ADC vs Microcontroller?
You could use this chip. It's intended to be used with a radio transmitter and receiver (detail to follow). The chip has 5 inputs all of which can be transmitted and received/decoded individually. You only need one input/output so this should be relatively straightforward.

I'd use them with radiometrix modules like these - they are called TX1 and RX1

You don't need to convert the 4V into a digital number - just regard the signal as being "on" or "off" and feed it into one of the 5 input pins on the TX2 encoder. The receiver/decoder has an output pin that will minic the sensor pin.