4-6 minutes is not too long at all, especially for a public speaking class. I volunteer with an English immersion group, and after only 4 days, all the students have to give a 7-10 minute presentation on a topic of their choice. At the beginning, they always think this is too long, but normally they are still talking after 10 minutes and have to be stopped!
As far as a subject is concerned, choose something you are genuinely interested in - it will be much easier to speak about something you already know about and are enthusiastic about.
To keep their attention - make lots of eye contact, smile, sound friendly, be enthusiastic and don't panic!
- For Teachers