Answer
Four years was the duration of Arkadiusz Milik's contract with Ajax. The Polish striker joined the Dutch club in the summer of 2014 from Bayer Leverkusen and quickly established himself as a key player, scoring 21 goals in his first season. He went on to make a total of 113 appearances for Ajax, scoring 47 goals, before leaving for Napoli in the summer of 2016.