In my experience, new adult dancers always think it is going to be easy. The reality hits early and hits hard. We teach American style and start with the rumba. A good dance to experience partnering for the first time. Just a few steps and lots of practice to music. Unlike some schools, we talk about proper technique right away. We tell them we don't expect them to get it but it is important that when they hear it again down the road, it isn't all new. So 'toe leads' on forward steps. Proper hold, heads up etc. For the last 5 minutes or so, we teach the merengue. Everyone can do it and they leave with smiles on their faces and a feeling that they can learn to dance. No one practices so the next lesson we spend 15 minutes reviewing and then we move on to the next dance. In a six hour beginner class, we teach rumba, tango, swing and salsa. These are the dances most requested. Tango is the most hated because it is so difficult but it introduces them to travelling around the dance floor. Adults can be difficult to teach. The men often have unrealistic expectations and they can get really miserable. I always stress that the beginner class is an introduction only.
As you know, four hours is but a drop in the bucket. Don't try to teach too much, 2 or 3 steps is all they need. They do not get bored if they are busy so keep them dancing.