Your name usually helps... what you've done in your previous life - straight from school or if you used to work in tesco... what you are into, sports / gang banging etc... you want to join x cap badge for y reason... you will be surprised how quickly 4 minuties go once you get going and one or two jokes always helps to lighten the mood.
Just dont say one liners icebreakers are there to see if you actually have some confidence about yourself...
Could be worse I had a guy talk about a BIC pen for 15 mins...
When you're not used to speaking in front of people it is easy to talk too fast, or too quietly. Always take a deep breath and direct your voice into the audience. You are communicating information, make sure they can hear and understand that information.
Keep it light and funny they don't want to be bored. It is also a bonding exercise, you are making common ground with some of the intake within the audience.
Obviously, start with your name (and nickname and why) and where your from, if it's a small village, tell them it's 10 miles north of major city etc. so they have an idea of the area you come from. Tell them about your family, Mum, Step Dad who was in X Regt, an older brother who is an electrician and a younger sister who is in primary school and a pain in the arse!
Tell them about your hobbies and interests. Rugby/football teams, ACF, favourite internet porn sites etc.
Why you are joining the Army and the Regt of choice.
It's always good to nip down the library to do some research on the regiment you'd like to be in so you can talk about why you want to join it and also give a little bit of history on the regiment too.
If your life story is boring you can add. They must be believable and have no consequences. I heard one bloke talk about a job in Birmingham. Later he told me that he had never been to Birmingham. It may be a boring lie but it kept him talking and nobody cared enough to ask any questions. The ideal lie.