Fourth Perk type advantage?

This indicates their weapon proficiency. You unlock weapon proficiencies by gaining weapon experience getting kills with that weapon. This will unlock proficiencies such as Impact, Focus or the ability to use two Attachments.

I suspect you are seeing the weapon perks which are gained by leveling up a particular weapon (If you hover over it in the create a class you can see a 'weapon level' near the picture of the gun). Here's a list of the cool stuff you can add to your gun.

• Kick (reduces guns kickback)
• Impact (increased damage penetrating surfaces and players)
• 2 Attachments (allows you to attach two attachments)
• Focus (reduce flinching when hit)
• Breath (hold breath like a sniper even with an assult rifle)
• Stability
• Rapid Fire
• Speed

But there is a way to get additional perks.

The 'Specialist' kill-streak.
Instead of traditional kill streaks or non traditional support kill streaks, the user has the option to pick up additional perks as kill streaks. Its an interesting trade off that can pay dividends if you get the correct set of perk synergy.

1. 2 kills unlock a perk
2. 4 kills unlock a perk
3. 6 kills unlock a perk
4. 8 kills unlock all perks