Working all of the major muscle groups is a must. So, just make sure you work all of your muscles equally, not just putting all of your focus on abs.
4 situps is a good start, but try working yourself up 2 more situps a day to 6 on day two, then 8 on day three, and so forth and so on. If you do too many, and your abs start to ache, just stop for a few days, and try it again with a lower number of situps. Eventually, you'll build up abs.
Leg lifts also work well for toning. Lay flat, with your arms at your side, and lift each leg off the floor about half a foot, and hold it there for half a minute. Try that one out, too
Finally, there are numerous machines out there, so join a gym, or get one of those home-gym doo-dads.
As always- None of my statements are evaluated by the FDA.
My advice is not intended to diagnose, treat, cure or prevent any diseases unless prescribed by a health practitioner. All information is provided for educational purposes only and should not replace the advice of your medical doctor.
(but your medical doctor will probably end up killing you