Roy Noji of PolarStar sent in a good number of photos of their upcoming HPA Gearbox replacement system. The photos below show the HPA system that you can install on existing M4 AEGs. The advantage of the HPA system is you can adjust the power depending on game requirements, no need to open the gun and replace the spring. The airsoft gun will be tethered to an external HPA tank, but then you can just have the hose run through your webbing to avoid entanglement.
They will be taking pre-orders by December 10, just in time for the Christmas season, so watch for news here, or check their website for announcements.