They can do it it is just they are designed for regular speakers.

That is a little misleading. You are correct that you need to balance the ohm load. However, if you have 2 component speakers (left and right) on the front that are 4 ohm each that means the front is loaded with 8 ohms...hence you need an 8 ohm sub on the rear channel if it is bridged. If I were using the rear in stereo, I would agree with you that 4 ohms on each right and left would be appropriate.

This configuration is straight out of the JL 300/4 owners manual.

So I have talked to a few new people and there saying that you will be fine. So I guess I am wrong on that aspect. Enjoy your system.