There's no difference in the stomach area. The baby is the size of a period at the end of a sentence. Pretty hard to imagine we were all that small at one time. The uterus is also still hidden behind the bladder and at about 5-6 weeks begins to move up and forward, pressing on the bladder which causes frequent bathroom trips. It not until the 3rd month that the top of the uterus is the same height as the bladder. For women who are carrying their first child, they usually don't see any changes in the lower abdominal area until around 14-16 weeks of pregnancy. For women who have already had a child, they tend to see changes around the 10-12th week. About the only thing that is different at 4 weeks, is a missing period but by the 5-6 week a lot of other symptoms take hold--morning sickness, tiredness, sore breast, lots of potty breaks, swelling of breast tissue, constipation, increase in cm, changes in blood vessels--they're more noticeable due to increase in blood volume and the cervix (which is normally pink) turns bluish/purple.
