Back in 2013, I decided to test if it was possible to use one keyboard as an input device for multiple players at once. The game was going to be a simple party game and the interactions with the keyboard had to be the main focus of the game: the keyboard is both what it's going to make you win and what makes you lose.
The game itself is easy the players just have to collect coins while running away from the lava, the level is shrinking continuously and at the end, the player with more coin is the winner. The difficulty relais in the power-ups that fall with the coins, those are going to switch the player camera or switch the commands for the players.

It was only when the game was nearly finished that I realized a huge technical problem that was probably going to kill this project: keyboards have hardware limitations on the number of simultaneous inputs. The letters, in particular, are designed to support only one press: this means that all the shift+a or ctrl+shift+alt+7 are well supported because the special button have a dedicated input line, but A+G+K+L for instance, can fail depending on the keyboard because A and L share the same hardware input line, if one of the two is pressed the other is ignored. This is something that during a normal use never happen, but as soon 4 players are using one keyboard, the amount of dropped kills the game.
It became obvious to me how important is to manage input correctly (always manage both the down and up inputs for button pressing!), it was a test nonetheless, I wasted my time in worst ways XD !
Even if it needs many adjustments the game is still playable. So again: you need 4 players and just one keyboard (better if a good one).