plural forms - a computer vs computers

1. 
   

   I can use computers.

   
   


2. 
   

   I know how to use a computer.

   
   
   

Why?

And what about the followings. Which of these is the natural sentence to say?

3. 
   

   He can teach how to use a computer.

   
   


4. 
   

   He can teach how to use computers.