Forget the dual 4" ports, as said, they occupy too much volume in your fixed size box. WinISD says that with 500 watts input, the single 5" PVC sweep elbow I posted will have a maximum airspeed of 17.9 m/s at just below tuning, provided you have a proper high pass filter set at tuning.
You have to make a compromise somewhere... and I think that going with a single 5" round port that's curved is about the best, lower air speed than a single 4", and takes up less of your fixed volume than a pair of 4" ports. There is no restriction compared to a pair of 90 degree elbows or 45's, the cross sectional area is constant and the curve is gentle.
"What the heck was that crashing noise?" Me: "oh, just tossed the shampoo bottles off the shelf on the opposite side of the house. "