4 hands massage is a massage where two therapists work on you at the same time, mirroring each others' movements. For example, both your legs or arms are being massaged at the same time. Or one therapist works on the right side of your back while the other works on the left.
To synchronize, one therapist takes the lead and the other follows. It should feel like they are doing the same thing simultaneously.
It helps if they have a similar style and touch and have worked together before.
When done well, 4 hands massage feels like you are completely enveloped in sensation and can be quite a wonderful experience. It costs twice as much because you have two therapists, but if you're a spa junkie and like to try anything at least once, you should definitely try it.