There are only four canopic jars—hence, Four Sons of Horus—even though there are more than four vital organs in a mummy (or a living human body). The heart got to stay in its place because it was considered the most important organ. And ancient Egyptians didn't think the brain of a mummy was worth anything, so they actually pulled it out through the mummy's nose and threw it away when they were done. Gross.
If you're ever bored, you can color your own (two-dimensional) canopic jars.
Each son has a pretty nifty name. Imsety means "Kind One;" Duamutef means "He who praises his mother;" Hapy means "Runner" or "Jogger;" and Qebhsenuef means "he who gives libations (liquid offerings) to his brothers." Which one would you want to befriend?
The ancient Egyptians sometimes thought of the Four Sons of Horus as being in the sky, like when they helped the king go to heaven in the Pyramid Texts. When they were in the sky, the Four Sons were stars, and specifically the four stars in the handle of the Big Dipper. That means you can say hello to them every night!