How much money will you get after 4 years in the military?

Whatever your position is it varies. It also depends if you receive a enlistment bonus. And if you are deployed, you get more money in a combat zone.

But if making money is your main reason for joining the military, I wouldn't recommend it.

Here is a current pay table:

http://www.dfas.mil/militarypay/militarypaytables/2009MilitaryPayTables.pdf