wait, you're saying that the RFs are 8 ohms in a single coil?? I didn't think they sold them like that. If they are, the thing to do is wire them in parallel to the amp, bridged across the two channels (both positives connected to the outside positive, and both negatives connected to the outside negative). That will give the amp a bridged 4 ohm load.
There is no way to "bridge your subs down to 4 ohms each", a 8 ohm sub is an 8 ohm sub and can't be changed.