One of the highlights was the incorrigible Damien Darhk as played by Neal McDonough. Darhk was a presence that's been sorely lacking since Slade Wilson/Deathstroke in season 2. Every time he was on the screen, Darhk brought a gravitas to the show that for all of its gloominess and Gotham-lite demeanor often fails to have real stakes. Darhk's often playful demeanor is always laced with the knowledge that he could just as easily kill you as he could shake your hand. After the disappointingly bland Ra's al Ghul in season 3, Darhk was a welcome presence. Darhk and his henchmen (referred to as Ghosts) started off the season strong, but soon became the regular type of cannon fodder that Arrow is known for (they're highly trained operatives that constantly resort to fist fights despite always having military-grade assault rifles and other weapons on them). This was no more jarring than during the season finale when a small army of Ghosts backs up Darhk in his battle against Oliver and a mob of angry citizens. The Ghosts could have and should have mowed them down.
Arrow's biggest problem is its narrative consistency. The Darhk storyline was the strongest of them, but it often took a backseat to other less compelling tales (Thea's bloodlust, Alex, the island flashbacks, the heavy handed Felicity/Oliver romance, the resolution of the tombstone reveal, the ridiculous nuclear missile hacking). Less of a mess than season 3, but still not cohesive enough to be memorable.
The whole angle with the tombstone was poorly executed and not well thought out. Laurel being the one who died did not carry any narrative weight with it because she hadn't been really important or meaningful to the show since season 1. The one who would've had the most impact being killed was Felicity. Despite being a fan favorite, her being killed in Darhk's conquest would've been the shockwave that rippled through Arrow. Instead, it came across as more of a whimper.
I know Arrow can be great, but it often struggles with finding its own footing. Season 4 ended with no hints of what's to come for season 5, so I really hope the writers dig into Green Arrow's history and bring some really compelling stuff out because as it stands it isn't looking so great.
It wasn't all doom and gloom, though. We got one solid episode with this guy, even though it was one and done. 
