The "standard" RC protocol is to transmit bursts of AM pulses[1], the relative positions of which are translated by the receiver into variable-width control pulses that get sent to the servos. It should be fairly straightforward for the smartphone app to send pulses out the headphone jack, which are then used to key a simple RF transmitter on a standard RC control frequency. Then, in the car, you'd use an off-the-shelf RC receiver and servos to control the car.
A standard RC servo requires a control pulse that has a variable width of 1-2 ms (sometimes this is extended to 0.5-2.5 ms) and repeats at a 20-50 Hz rate. A basic RC car might have as few as two servos, while a complex plane might have six or more, each of which needs its independently-controlled pulse.
Rather than sending variable-width pulses over the air, which would be subject to noise and distortion of various types, the transmitter instead sends a series of fixed-width pulses (on the order of 0.5 ms or so), and it's the spacing among these RF pulses that becomes the width of the servo control pulse at the output of the receiver.
For example, a six-channel transmitter will send 7 pulses at a time. The time between the rising edge of the first pulse and the rising edge of the second pulse is turned by the receiver into the control pulse for servo #1. Similarly, the time from the second RF pulse to the third RF pulse becomes the control pulse for servo #2, and so on. After the seventh RF pulse, there's a relatively long gap (10 ms or more), which signals the receiver to reset its decoder back to servo #1.
[1]: I might be wrong about the modulation. Certainly, the earliest systems used AM, but indications are that modern systems are FM.
This is a harder problem than I think you realize. The basic Loc8tor system works primarily by virtue of a directional antenna in the receiver, and it doesn't really give you the position of the tag directly, it just indicates what direction it is from the location of the receiver, along with a very rough estimate of how far away it might be.
If you want to create a mesh of automatic receivers, each of these receivers will need to do direction finding, either mechanically by physically spinning its antenna in a circle, or by using multiple antennas and electrically "spinning" the reception pattern. Neither method is going to be simple to implement or particularly low-power.
It is only by combining direction information from two or more receivers (plus knowing exactly where these receivers themselves are located) will you be able to derive an absolute position for the tag.
Best Answer
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.