Ambient backscatter uses radio frequency signals available in the environment (e.g. radio broadcasting, television or mobile telephony) to transmit data effectively leading to significant energy and cost efficiency increase. This paper presents a novel wireless tag, which for the first time utilizes 4-pulse amplitude modulation (4-PAM) technique to modulate the ambient backscattered FM signals in order to send data to a nearby low-cost software defined radio reader. The tag is based on an RF front-end that uses a single transistor controlled by an ultra low-power microcontroller. The microcontroller includes an analog-to-digital converter (ADC) for sensing and a digitalto-analog converter (DAC) for RF front-end control. A proofof-concept prototype is demonstrated in an indoor environment with the low bit rate of 345 bps and power consumption 27 µW. It operated using a real FM station at 34.5 Km away and the tagto-reader distance was tested at 1 m. The value of energy spent in this modulator was 78.2 nJ/bit at 345 bps and 27.7 nJ/bit at 10.2 Kbps.