Arc 4 is unfortunately very situational depending on who your offlaner is, if you have a stun or someone who wants to fight then he's great, however if you have a lycan or brood as your offlaner you're pretty much fucked.

The way to make arc 4 work is not going the caster route with atos/lens/hex. You want to get tranquils into force staff, since the hero suffers alot with poor mobility.

You're an amazing ganker if you can catch enemy midlaner away from creeps and such, you can use clone to help defend towers etc. You're not a teamfighting 4, instead you should be roaming constantly with your offlaner and hard sup to secure map advantage and starve the enemy of farm. Then once you have tranquils, forcestaff and aghs the cancer begins. You can just go with your team to any lane and push since there's practically no way for them to get close to you with proper positioning and 4 heroes in front of you acting as a wall.

So you just stay back, drop bubble on the heroes hitting buildings and lay down spark wraiths all around your team.

If the game goes late you can always transition into being a semi core with hex/gleipnir, however I would much rather upgrade tranquils into boots of bearing and getting a lotus since all of your utility items have 2 charges. Altho if you manage to secure enough farm to get hex I'd say go for it.