Well if you have n, forces, then I would define the average of these forces to be , . And in this sense, no, the resultant force isn't an average. The resultant force is quite simply a force whose magnitude, direction and position reproduce identical statical or dynamical conditions as the single forces that were considered. That means that if 4 forces accelerate an object with an acceleration of 50m/s^2 in the negative x direction, with 0 moments about all points, then the resultant force of these 4 must also produce an acceleration of 50m/s^2 in the negative x direction, with 0 moments about all points. However if you are talking about the direction of a resultant force it is SLIGHTLY like the average direction in some cases. If you have two forces in equal magnitude. On at -45 degrees, and the other at 45 degrees, then indeed the resultant direction is the average of this... 0!. Or if they were at 50 degrees and -40 degrees, the resultant direction would be 5 degrees
Indeed, if you have two forces acting on a body, and they are equal in magnitude, opposite in direction, and colinear, then the average of these forces is 0, and this just happens to be the resultant force also. But in general, the resultant force is not an average.
To replace 4 forces with a single resultant force you need consider a few things.
1) Split your 4 forces into x components and y components, and then sum the x components and sum the y components. The y component of the resultant force is equal to the sum of the y components of the other forces. Same for x.
If you draw these x and y summations, you will get a triangle triangle, and you may use pythagorus and trigonometry to calculate the magnitude and angle of the resultant force.
2) Consider the moments of your force. If your 4 forces create moments about a certain point, then your resultant force must also create the same moment about the same point. You need to choose your point carefully... try to choose a point that most of the forces pass through, because if a force passes through a point, it creates no moment about that point... the more forces that pass through the point you choose, the less work you have to do! Given the magnitude and direction of the resultant force, you should be able to find it's position by calculating it's distance from that point, if the moment it creates is to be equal to the moments created by the other 4 forces.
PS: You said your resultant is in a direction in the opposite sense to the other forces :S... That doesn't make sense! If you have 4 forces all in the same direction, then the resultant will be in the same direction with a larger magnitude...