The Gopro 3 uses the Ambarella A7 chip, the 3+ uses the Ambarella A7L which only offers improvement in wifi and battery consumption.
The latest Ambarella A9 chip was announced very early this year so you can expect Gopro to be doing testing on it now for their next Gopro 4.
http://www.ambarella.com/products/consumer-hybrid-cameras.html
That should be able to capture 4k at 30fps. I think the only problem is battery life, heat and micro sd card speed that is constant and fast enough to record it and fitting everything in to a Hero 3 size and form factor.
But seeing how a Samsung Note 3 can already record 4k at 30fps, Gopro is playing a catch up game now.