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
It is normal to see pixelation when zooming in past 100%. It's also normal to not see pixelation.
To zoom in to 100% means there is a 1-1 correspondence between the image pixels and display pixels. When zooming in past 100%, there are "gaps" between pixels that need to be filled in for display. (May be thought of as "stretching out" the pixels to fill the screen.)
Filling in the "gaps" between pixels is known as interpolation. Some methods result in a pixelated appearance (nearest neighbor). Others won't (bilinear, cubic, sinc/lanczos). The interpolation method can be selected in most image viewers. Some programs may refer to them by complexity instead of name (fastest, fast, good, better, best, etc).