I emailed VetUK about them a while back and they are perfectly safe, no risk of overdosing, safe for all ages of cats and safe even for pregnant cats and newborn kittens so nothing to worry about at all and no negative side effects
It's feline pheremones that promote a feeling of wellbeing in the cats so won't do any harm to your other 3 cats. I use Feliway on mine and it sees to work well.