R/C remote controllers use channels (4 or 6 is standard) but that is not related to the number of motors. Its the number of controls you have. One for throttle, one for yaw, pitch, arming the quad and so on.
Technically you can control the ESC from any pin on an arduino, but the hobby industry has a standard pin configurations so programs like MultiWii will be compatible with there hardware.
If you have 4 motors its just 4 wires signal wires + GND's . In MultiWii you would pick the QuadP or QUADX option because you have 4 motors. Once you have all the options in the software selected, upload it to the board. Then open the MulitWii processing program, it will help let you know what options you pick, how to wire the motors to the board and so on. Make sure you have the motor rotations correct.
Without propellers you can pretty easily play with it before trying a real test. If something doesn't respond properly no harm will happen.
If your going to do all this be prepared for the worst. Its not unheard of for people to obliterate there quads within a few minutes of take off.
You should do some research on Quad-copters on hobby sites to get a feel for what's required. For cheap equipment Hobby King is ok.
If you want to manually control the ESC without a R/C remote you can use the Servo commands as a basic input. Most ESC need to be programmed before use, read the instructions on yours. Its mostly the min/max values it will respond to. If you can get that done (you need to do it quickly for it to accept it) then you can use servo commands to rev your motors.