Tabletop Simulator is currently in Early Access and can be purchased at the official distributors below:
Steam, Humble Store, Greenman Gaming, Bundle Stars Store, Desura, MacGameStore, and Amazon.
already own the game, you will either only receive 3 copies (Humble), or you won’t be able to purchase a 4-pack through Steam because you
already have a copy. So what you need to do is purchase a 4-pack as a gift to someone else so they can distribute it. This applies to both Steam and Humble.