It is possible for a 4-week pregnancy to not be visible on a scan machine as it may still be too early to detect. The embryo is very small at this stage and may not yet have developed enough for detection. However, if you are experiencing lower abdominal pain, it is important to consult with your healthcare provider as there can be various reasons for this discomfort. Some common causes of lower abdominal pain during early pregnancy include implantation cramping, stretching of the uterus, or gastrointestinal issues. Your healthcare provider will be able to evaluate your symptoms and provide appropriate guidance and care.