Take a look at this book - take particular note of the chapter on sizing your power system.
EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT
http://www.rcuniverse.com/forum/m_7100376/tm.htm
You can do some first cut approximations using these calculators:
http://www.electrifly.com/powersyste...tem-index.html
Most are based on single motors. you will need an estimate of the all up finished flying weight. If you are going to have 4 motors then you will take that weight and divide by 4 to get how much each motor is supporting in a power to weight or watts/pound ratio.
Typically you will want to be in the 75 to 125 watts per pound range depending on what kind of performance you expect.
Take a look and then ask some questions.