Clownfish are pretty territorial creatures. Worst case scenario is that the original clowns will kill the new clowns. When this happens is largely dependent on the species of clown (some are known to be more aggressive than others), the age of the clowns (as they approach maturity, their territorial aggressive tendencies increase), availability of resources, and space. A 40 breeder might work in the short term, but is unlikely to be a long term solution for four clowns in most cases- it will always be a risk. Many people are able to keep multiple clowns together in these smaller tanks for a year or two if they are lucky, but generally harem tanks need much more space and resources to offset that aggression. I personally don't like to have a time table on my pets and I really don't enjoy coming in to find dead fish, so I avoid situations where there is a large risk of a bad outcome in the future and try and stock within the constraints of my personal situation (tank size, current inhabitants, etc.). Your decision to house 4 clowns is largely of how risk adverse you are.