Try both of them hooked up together onto just one set of binding posts (like the right speaker) and see if your gain is manageable. If that works then adding another pair should not be an issue.
My question about wiring up a pair of subs and what to do with the 3 wires resulted in this response from REL:
"Thank you for reaching out. Either a signal ground or earth ground will be fine for connecting the black High Level wires to, though if possible the earth ground screw would be preferred. If you experience hum from the subwoofer when connecting the black wires to this screw, then you could instead try the signal ground and see if this yields improved results.
We recommend starting with both the red and yellow High Level wires connected for each subwoofer, and in most cases this will result in excellent performance and usable signal levels. However, if you find that the subwoofers have too much output, where barely raising their Level controls already results in them overwhelming the output of your main loudspeakers, then you could try disconnecting one of either the red or yellow wire for each subwoofer. This will lower the input signal gain roughly by half, and will allow for greater flexibility when setting their Level controls. The red and yellow wires are functionally identical, though we generally recommend leaving the red wire connected for the right channel subwoofer and yellow wire connected for the left channel subwoofer in these cases. "
You can always ask REL this question directly for 4 subs. I feel confident it will be okay on the high level.
