Definition of a 4 way situation
A 4 way situation or 4 way pot is said to be a move played when there were 4 players present. A 4 way pot can be played for several reasons. One of them is that all players have a very strong hand. It is also possible that the action was passive preflop. When one player limps and another decides to call, most of the time the blinds (small and big blinds) will also be in the hand. Situations played 4 way will be rather rare situations unless you find yourself in passive casino games.
A classic example of a 4 way pot
A recreational and curious player limps in UTG position. The player on the button has a speculative hand (say 5-6 suited) and also decides to limp. The player in the small blind then only has a small amount to add to the pot to go to a flop and the player in the big blind position has already paid his blind and will go to a flop by default. So we know that the pot will be played 4 ways. In this type of situation, players will tend to play to the strength of their hand and bluff little.