Just like any other form of massage, the most immediate benefit is the relaxation.
This relaxation can lead to many other benefits, including less mental stress (which in turn relieves many illnesses or pains).
Is a four hand massage any better than a regular massage?
It depends on what benefits you are seeking as to whether a four hand massage is what you are truly looking for.
Four hand massage also promotes healing — just as other forms of massage do—by increasing blood flow and circulation throughout the body.