A first person shooter with 4-player co-op for Wiiware? It's not what you have come to expect from the service, but that is just what Hudson Entertainment's Onslaught is. The graphics aren't exactly what you would expect from a FPS these days, but the size limitations on Wiiware obviously take their toll.
Here is Hudson's short description of the story:
"In the distant future, contact with a distant research colony is lost and your team is sent in to find out what happened. Upon arrival, you're attacked by overwhelming numbers of mysterious creatures. It's up to you and your squad to eliminate the threat and discover the truth that lies deep inside the colony."