how much stuff can a 16gb ipad 4 hold?

If you want hundreds of songs, dozens of apps, and a couple of movies: 16 GB

If you want thousands of songs, hundreds of apps, and several movies: 32 GB

If you have more money than time to think about this any more: 64 GB

This may help: How much content will fit on my iPod or iPhone?

It applies to iPads as well.

About 13.7GB.

That would be:

~10 movies

~2500 songs

The rest is hard to say as app, photo and book sizes can vary.

16GB vs 32GB vs 64GB: Which new iPad storage capacity should you get?

http://www.imore.com/2012/03/08/16gb-32gb-64gb-ipad-capacity/

How much content will fit on my iPod or iPhone?

http://support.apple.com/kb/HT1867

 Cheers, Tom 😉