Babies are still very small at four months. They usually weigh less than a pound at that point. Of course you aren't going to have a great big belly like someone who is eight or nine months along. You're at the point now where some women are just beginning to show, but not even wearing maternity clothes yet.
Sometimes miscarriages are noticeable, and sometimes they are not. A woman might have a miscarriage without even realizing that she was pregnant if it happens very early on. Later on in pregnancy, it becomes far more apparent.
If you are pregnant, you should be taking this time to look into fetal development. There is an awful lot that you should know about your body and your baby's development by this point, as well as many other things. Try looking for some books like What to Expect When You're Expecting. If you don't have the money to buy new books, no worries. They're often easy to find at garage sales or at your public library. There's also a lot of information available on the internet. You can look here:
http://pregnancy.about.com/od/fetus/ss/ninemonths.htm for a quick overview of what is happening with your baby each month. And you can look here:
http://pregnancy.about.com/od/pregnantbody/ss/monthlymom.htm for a quick overview of what is happening with your body.
This is stuff that people should ideally know at least some of before they even get pregnant. If you have not been using these past four months to study this kind of stuff, please use the next five or more to do so. Some of this stuff can even be a matter of life and death (as far as what vitamins you need to be taking, what you should be avoiding, etc.). Even after your baby is born, there is still a lot of stuff to know, and chances are, with a newborn, you might get a lot less reading done then. So now is really the time to do it. You should also be seeing an ob-gyn by now. It is never too early to start learning and getting involved in your child's life.