Question

Which 4 elements make up the bulk of living matter?

Answer 1

"Let's start with the basics:  you have probably heard that your body is 90% (or 85% or 95%) water.  In actual fact, water is (probably--unless you are obese) the majority of your body weight--but nowhere near 90%.  The best estimate I have seen is 57% (Body water).  So that puts hydrogen and oxygen in the top four.
 
As I mentioned, extremely obese people can be less than 50% water.  Fat (triglycerides) is composed of carbon, hydrogen, and oxygen.  The majority of the mass of a triglyceride is carbon--chemically it is closely related to biodiesel.  So that puts carbon in the top four.
 
Finally, amino acids which make up proteins (which make up muscle) require nitrogen.  That makes the top four.:  Oxygen, Carbon, Hydrogen, and Nitrogen.
 
But wait!  What about bones?  Aren't they calcium and phosphorous?  Bones are really heavy. 
 
It turns out that calcium and phosphorous are indeed the next two most abundant (by mass) elements in the body, but where nitrogen is about 3% of your body mass, calcium and phosphorous are 1.5% and 1.2% respectively. 
 
You can find the full breakdown of the elemental composition of your body here: Abundance of the chemical elements"