The real issue with the FP4 is the camera. Ugly pictures, and under bad lightning its like taking picture on android a decade ago.. Albeit the pixel number is enough.
If you want an opensource phone, you end up with crappy photos unless you have a camera team of 500 people to rewrite that stuff, and a big team of lawyers to deal with all the patent licensing you're going to have to do for the techniques you use.
Camera stacks on modern phones process many raw frames to make a single photo - sometimes as many as 100.
Collecting those frames is done in a second or so before and after the user clicks the shutter button. Thats a data rate of ~15 Gigabits - far faster than most phones can write to storage - and anyway would you be happy for each photo to take a few gigabytes of storage space until it was processed?
Therefore, the raw data needs to be processed (or at least preprocessed) in realtime. You can't store it and do it later.
And to do that, you typically need custom silicon - all the big phone manufacturers have ASICs dedicated to image processing.
And to make the trouble even worse, you often need to use data from some of the raw frames to adjust camera parameters for the next frame - for example adjusting amplifier gains. See patent US9196027B2 for an example of the sort of thing that is done 'in the loop'.
At a minimum, the GPU needs to be able to generate image scale pyramids, align them using optical flow, have a sensor noise model to detect misalignment, and take a running-mean of every non-misaligned pixel in each frame seen. This technique only needs enough ram for a handful of frames.
That should get you a good chunk of the way to a decent image.
As usual, the actual math is quite simple in python, but when you need it to run at 15 Gbits you'll be spending a really long time optimising assembly code for whatever GPU you're using...