I can't understand something...

In a player action window, say during a challenge, every player starting from first player can take only one single action each?

No, players can take actions until all players consecutively pass.

until for example, in a game of 4 players, 4 passes are declared...

If i got it right!!

Note that that is 4 passes in a row - one by each player. If 3 of 4 players pass and the 4th takes an action, the "4 passes" count starts all over again.

got that, thanks mates