addVectors( a, b ) {

		this.x = a.x + b.x;
		this.y = a.y + b.y;
		this.z = a.z + b.z;
		this.w = a.w + b.w;

		return this;

	}
Adds the given vectors and stores the result in this instance.

@param {Vector4} a - The first vector.
@param {Vector4} b - The second vector.
@return {Vector4} A reference to this vector.