Well I'm pretty sure playback is only software disabled as well, as it can playback 4k from the external rec. If interested Sony could very well make the f5 a full 4k internal camera via firmware.
The comparison with GPU or CPU chips is a bit wrong at least in the beginning of the lifespan of the chips. Most of the time when producing a new chip on a new manufacturing process not every chip is 100% usable but before thrashing them they get sold as a cheaper version with for example 2 non working cores disabled. As manufacturing matures the yield gets better and then you get nearly 100% working chips. And then they keep selling the cheap version. Therefore the "hack" of re-enabling disabled cores via software, soldering or something else doesn't always work. But on the F5 its pure marketing decisions. I hope they just sell a 4k upgrade option, they are lucky at the moment that enabling of 4k playback of 4k isn't yet working so they have a chance of selling it. Otherwise v5 must be really good to convince people to forget about 4k recording and upgrade their firmware to v5.