The +4 dBm designation indicates that a full modulation signal will just reach +4dBm rms max. The line level input level at the transmitter can be attenuated by different factors depending on the pins selected (pin 4 or 5) and the resistor to ground (pin 3 or 4). The gain pot also can vary the applied gain also. To sum up, there is no fixed relationship between the input level to the transmitter and the output level from the receiver other than the +4 dBm setting is the maximum output level you can see. If the transmitter is set up so that you never activate the limiter, then you will never see the +4dBm. If you have 20 dB of headroom between your maximum input level and the transmiter limiter, then you will never see more than -16 dBm. My guess is that you are being pretty conservative on your input levels and could run them substantially higher; this will bring your output levels closer to the max indication and also improve the signal to noise ratios and noise ups.