Say a published paper has 4 authors. How many directly contributed to writing the paper? As in the question, let's say that a paper has four authors. Clearly, all authors contributed something to the work. However, can I deduce that only the first person wrote the physical paper? Or do authors normally write different sections of a paper? For instance, is it possible that author 1 writes the first part of the paper and author 2 writes the second part of the paper? Or, can it be the case that the first author listed contributed the most to the work stated in the paper but did not write the actual paper? 

Main questions: 


Given a paper with multiple authors, on average, how many people wrote the physical paper? 
Is the first author always the person who wrote the physical paper?
This depends on the field and what is customary there. In mathematics you can assume (generally) that all contributed equally. In lab sciences it may not be the case. It is customary in some that the PI who created the lab is on all publications as a co-author but may not have contributed anything to the writing and possibly very little to the ideas. But the PI makes the work of the lab critters possible, so gets on every paper. In some such fields, it is also customary that some fairly low level technicians get on some papers, though didn't contribute to the writing, but they managed the experiments for the actual authors. 

Ideally, papers in which people don't contribute "equally" (whatever that means) have a Contributions section to detail the contributions of each as well, possibly of some who weren't listed as co-authors. 

The actual writer of the paper may be listed first or not. Sometimes the PI is listed first and sometimes last. 

But be aware that intellectual work is hard to measure. Someone may have only spent a few moments thinking about the problem at hand, but provided the crux of the solution that was then written up by others. Flash of insight. 

Someone who writes a lot in the field in question can probably answer questions about what is customary in that field. In CS we tend to list authors alphabetically and don't worry much about such things. But we also tend to list only people who actually contribute something meaningful.