That tank is way to small for 4 clownfish. You can have other fish but not another pair of clownfish. clownfish are extremely territorial so would could have happened is the other pair of clownfish were in there territory and they could have killed them or they could have tricked them to go into the anemone as food for it if they were hosting it. However if there is no dead fish bodies or white goo in the tank I am puzzled.
Although what kind of filter do you have?
The_Fish_Man otherwise known as Scooby
"Each tank is a different world....... Bored of yours go into your fishes":p