#4 Kieran Tierney (Celtic)

Kieran Tierney is a 21-year-old left-back who currently plays for Celtic in the Scottish Premiership. He has also featured for Scotland's national team 12 times since making his debut in March 2016.
Although a hip injury forced him out of many games for Celtic this season, Tierney has still managed to play in 18 matches and register 5 assists in the process. Tierney has a turn of pace on him that leaves defenders lagging behind and if they fail to reach him in time, he can whip in a devastating ball for his teammates to score with ease.
Tierney has been with Celtic for the entirety of his footballing career, having progressed through their youth academy, to the U17s, and the U20s, before finally being called up to the first team in 2015, when he was just 18 years old.
Kieran Tierney would make for a great tactical fit at the Emirates, and could possibly even replace Kolasinac who has been largely lacklustre this season. The Scot has drawn comparisons to the likes of Andrew Robertson at Liverpool; if Arsenal could get a player like Tierney and utilise him the same way Robertson is at Anfield, they could do wonders.