Sni[p]r 4 is the 4th in the series of stickman shooting flash arcades similar to tower defense games. Actually it's the same engine as the one used in Turret Tower Attack. On the first they look exactly the same.
In Sni[p]r 4 you play stick sniper assassin who protects his base from tens of armed enemies who are approaching shooting with their guns and tanks. You get payed for every head shot you make, and this is how you earn points i.e. money. When you collect enough money, you can than spend them on upgrades which consist mostly of better weapons and things like gun stabilizers, better ammo and so on. It is possible to destroy a tank but it takes a lot more hits and you wouldn't be able to accomplish that with your basic gun. You should keep the tank away from the building. It is so powerful that it would take only a couple of seconds to destroy you regardless your health status at the time.
More less this is an endless shooter and eventually you die. The longer you survive the better, that's the whole point. It's actually quite an old game, it's a classic now.