If you run those in parallel isn't it about 3.4 ohms? That's pretty close to four. If you want to dial it in closer, you could drop a one ohm or so resistor in series with each sub to bring up the resistance of each as close to 8 as you can get them. That'd put the parallel configuration right at four ohms. I'm not sure if there are performance issues for the subs with doing that but electrically, it'll dial that circuit in where you want it. I'm sure someone with more experience playing with these sort of things will chime in with a more informed solution.