I think swarms of minions would be a threat to this party. In a recent one-off, I had two strikers and a defender (fighter, warlock, and ranger) go up against a controller, a few artillery, and a bunch of artillery minions. The minions were the biggest challenge--no threat individually, but they got a lot of attacks and whittled the party down.
Also, one leader isn't a lot of healing for four soft targets, since the leader only gets a handful of heals per encounter.
Also, one leader isn't a lot of healing for four soft targets, since the leader only gets a handful of heals per encounter.