It is common for a gestational sac to be visible without a yolk sac at 4 weeks 6 days of pregnancy. The yolk sac usually becomes visible between 5-6 weeks of pregnancy. However, every pregnancy is different and it's best to consult with your healthcare provider for further evaluation and reassurance. They may recommend follow-up scans to monitor the development of the pregnancy. Remember that early ultrasounds can sometimes be inconclusive, so try not to panic until you have more information from your healthcare provider.