hold off on that booster cap. Your receiver should work just fine at 4.9 volts. The fact that the steering servo is working means that the receiver is working.
This is what I'd check, in this order:
1. As Carbon Joe said, make sure the speed control is calibrated. Follow the calibration steps in the speed control manual. That ought to get you going if you haven't already done that.
2. I'm not familiar with the speed control you mentioned, so if step 1 doesn't fix your issue try reversing your throttle channel in the transmitter and repeat step 1. Some speed controls expect the throttle signal to be in a certain "direction" before they'll accept calibration.
3. If still not working, go into the radio end point adjustments for the throttle channel and make sure the endpoints for throttle and brake are set to the max. Then repeat step 1 and if necessary, step 2 again.
4. If you're still not up and running, check the plug from the speed control that goes into the receiver. one of the little terminals in the plug may not be fully seated, and that will prevent the speed control from getting signal from the receiver. The terminal you're concerned with is the one furthest from the edge of the receiver case when plugged in. That';s the terminal for the signal wire. The other two should be fine, or you'd get no lights on the speed control, and no steering. While you're at it, make sure the wire from the speed control to the plug for the receiver is intact... if the signal wire is cut or broken, the speed control will power up and you'll get steering, but no response from the speed control. Again, the "signal" wire is in most cases the one furthest from the edge of the receiver case when plugged in. I won't ask you to make sure the plug from the speed control is not in backwards, if it was you would get no lights at all and no steering.
5. I'm out of ideas at this point.
Hope that helps, let us know.