Well, the first one shows a JFET, so there is no 4th terminal.
Effectively, in a symmetrical JFET, the drain pin is whichever end of the channel is more cut off (i.e., more positive with respect to the gate, for N-channel), and the source is the other. For AC signals, this has the functionality alternating, which is why a JFET makes a poor variable resistor, at least when just hanging out there without any help.
(Usually, a "variable resistor" is made by grounding the source -- so you at least have some hope of a stable reference voltage for the gate -- and driving the gate from a voltage divider spanning from drain to the control input. This acts to linearize the response, to some extent, but it still suffers due to cutoff and constant current regions, for more than a modest resistance ratio. That is, you can go from perhaps Rds(on) to 4*Rds(on), over a reasonable range of drain currents/voltages; but you aren't really going to achieve 10:1 or more, even with small drain voltages.)
Alternately, since the modulation is digital anyway, it might simply be a FET of any type, driven into saturation (i.e., fully 'on', regardless of drain voltage), where Rds(on) is controlled to be the amount required for the modulation depth. Which, in an IC, might be a very long, thin channel (hence, high intrinsic Rds(on)), or a proper (overly large, or even discrete sized) transistor paired with a (more accurate) resistor.
The consequences of nonlinearity are unreliable (e.g., level-dependent) modulation depth, distortion of the modulation itself (if analog or multilevel modulation is required), and shifting of resonant frequency (because the load varies during a cycle of the RF wave itself). Using a saturated transistor and (much better -- lower capacitance, linear / ohmic) resistor solves both of those problems (assuming, again, that analog or multilevel modulation isn't required -- though in principle, you could use many switches in parallel, to control a binary series of loading resistors, that would eventually look analog enough if needed).
Tim
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