here are some rx specs showing voltage as low as 3.6 volts this is one of those thing that can be argued back and forth but for some reason i keep posting that link to flying giants which is talking spectrum rx at 3.6 volts.but here is a futaba spec and a gws spec of rx voltage
Futaba Lo and Hi Band 8 Channel PCM Receiver




All Mid Size / Full Size Helis



The Futaba R138DP is a 8 channel PCM receiver. Narrow-band spacing and dual conversion signal filtering combined to provide exceptionally clear, interference-free reception and amazingly smooth, accurate control. Designed for modelers who want to





use a single radio transmitter to fly several different model aircraft, especially pilots who are utilizing computerized radios with multi-model memory.


Channels: 8
Band: 72MHz
Conversion: PCM
Operating Voltage: 3.5-6.0 Volts
Range: 6,000+ Feet
Antenna Length: 39”
Spacing: 20KHz
Case Size: 2.5” x 1.38 x 0.88” (63.5x35x22mm)
Weight: 1.75 oz. (50g)

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GWS R-4PII/T Pico Receiver for Airtronics/JR Radios $19.20

GWS R-4PII/T Pico Receiver with Micro JST Connector for Airtronics/JR Radios
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GWS R-4PII/T Pico Receiver for Airtronics/JR Radios

This receiver comes with Micro JST connector and is designed for slow/park/indoor flyers. Servos and ESC must have Micro JST Connector to work with this receiver.

Specifications:

Number of Channel: 4
Modulation: FM (PPM) Single Conversion
Signaling: Positive Shift
Operating Voltage: 3.6 to 7.2 V
Current Drain: 5 mA
Weight: 3.6 g (0.13 oz)
Dimensions: 15.0 x.25.0 x 9.5 mm (0.59 x 1.00 x 0.37 in)
Range: 150 m (500 ft)