4K might be the next big thing in video, but for still photography that's just 8 megapixels, which is quite low for most cameras, and I think around the resolution of the iPhone. I have a Nikon D5300 with 24MP resolution, and I've seen other DSLRs get up to 36MP or higher.

And no matter what, if you zoom in enough, you will eventually get pixelation (anything over 100% will look blocky), that's just the nature of digital. Make sure that you judge the picture by looking at it from the intended distance and at the intended zoom level. Rarely will you look at a picture at 100% size on a monitor, and prints are typically viewed at a distance, vary rarely will someone take a magnifying glass to it.

It is possible that the .jpg compression is done at a low quality (high compression), which can create blocky artifacts (say 8px by 8px blocks) that look very similar to a pixelated picture. Using less compression (higher quality) will help, but increase the file size (that is the point of jpeg, after all!)

And no resolution will make up for a blurry or out of focus picture. That's often a bigger problem than the megapixel size. The law of diminishing returns really becomes significant as more pixels are crammed onto a sensor.