I'd suggest sticking with one pair and then getting other types of fish. They really do get aggressive as they get older. We have a 90 gallon tank with a pair of ocellaris clownfish that have been together for almost a year. We got them when they were small and now that they're older and bonded they're very aggressive towards other fish. Even with a bigger tank I'd think having two pairs of clownfish would likely result in one pair killing the other. Even if they do stake out different parts of the tank and avoid each other that means that the other fish in the tank will basically have nowhere to go without being harassed by clownfish. They really are a bunch of jerks as they get older.