I heard about some research a local company (that no longer exists) was doing years ago. They were looking into using a planetary configuration to achieve something like what you're asking for; 1 shaft in, four shafts out. The most detail I ever got was that they used a series of independent carriers which had a series of helical skew gears in each. They manufactured conveyor drives, what they needed a four shaft differential for and if they succeeded, I never found out in the end. An interesting proposal I thought, hope it gives you some ideas.