In "photos" on a macbook, there is an option to zoom in a photo up to 400 percent. Is it normal to see some pixelation when fully zoomed in?
400% is not just fully zoomed in - it is much more than fully zoomed in. In some cases it may be beneficial to view photos at 100% but I can't think of a use-case for viewing at more than that. To answer your question, it would be very strange (or completely incorrect and misleading even) if you didn't see pixelation at 400% magnification.
The iPhone 6's rear camera shoots 3264 x 2448 pixel photos.
If you zoom in 4 times, you're looking at a quarter of those pixels, which is 816 x 612 pixels.
Your monitor likely displays 1920 x 1080 pixels or larger. If you want to display a 816 x 612 pixel image on the entirety of such a monitor, it has to 'stretch out' these pixels so it fills up the monitor. If you assume 100% to fit the monitor exactly, 400% means you're showing 1 pixel of the photo on 4 pixels on the monitor- which would show as 'pixelated'
In short: yes, it's entirely normal
In regard to John's comment
Some clarification about the difference between viewing at 400% and zooming in 4 times:
Seeing as how your iPhone's camera shoots photos with more pixels than the monitor you're using, viewing the photos at 100% would mean they're 'zoomed in' in your monitor.
Zooming in 4 times would not necessarily create the same image as viewing at 400%. This depends on the monitor size and the starting point before zooming in.
This aside, zooming in and viewing at a certain scale will always mean a certain amount of pixelation if the number of pixels to be displayed is lower than the pixels the monitor consists of