It is not uncommon for a gestation sac to be undetectable on an ultrasound at 4 weeks and 4 days of pregnancy. At this early stage, the embryo may still be too small to be seen or the pregnancy may be in its very early stages. It is important to remember that every pregnancy progresses differently, and it is best to consult with your healthcare provider for further evaluation and guidance. They may recommend follow-up ultrasounds or additional tests to monitor the progress of your pregnancy.