Capture is 4k on the F65 and the Red, 3k (2880x1620) for the Arri Alexa. Despite all of that, often the finish a.k.a. master is 2k (2048x1152) for cost reasons.
Even when it is a 4k master, by the time it goes through the compression on youtube or Netflix, you've lost quite a bit anyway. Blu-ray is a different story--and that does make a difference.
But my point was, if you can afford 4k, get it. But high resolution does not always mean a better picture. That's what makes some TVs $2500 and other TVs the same size $800. Cameras are the same story. Spend your money on the optics, not just the number of megapixels.