Sniper Elite 4 is one of my fav co-ops with my buddy really fun game. Only sad, is that you can't do more then 2 people for co-op unless you do the special mode "Survival" Where you can have three people and basically play king of the hill with waves of germans charging you. You hold each point a little like Insurgency.
If you like Sniper Elite Series, Rebellion uses this basic controls and engine for several even better games for co-op
The Zombie Army Trilogy has loads of content and is basically Sniper Elite plus Wolfenstein.... Shooting National Socialists is just as much fun when they're undead.
The Strange Brigade is Sniper Elite crossed with Tomb Raider, and the Mummy, and Raiders of the Lost Ark movies. All stealth is removed and it's easier controls if your a controller player....(i'm not) Still this is high paced action and problem solving and some of the best action co-op I have played it's good with 2 people but 3-4 people and it really shines.
Also the voice acting is top notch and hilarious. The narrator commenting on your actions in particularly is funny.
Basically all the Rebellion Sniper Elite engine co-op spin offs have been quality co-op fun.