Most people don't even know they're pregnant at 4 weeks. I wouldn't worry.
My doctor also had me do a ridiculously early test, which was a waste of time and money, and also made me overly anxious when they couldn't see anything.
If you're using a private ob/gyn in Central, particularly a "name brand" one, be careful that he is not milking you for cash with lots of extra, unnecessary scans and treatments.
My first doctor rambled on about possible twins, etopic pregnancy, miscarriage, as soon as I found out I was pregnant, and sent me off for scans and tests that I later realized were not needed. He tried to convince me I was high risk, and prescribed painful, expensive injections. Even worse, he caused alot of emotional stress.
When I sought a second opinion with a less "famous" doctor, I realized that doctor #1 was just fooling around with me.
I stayed with doctor #2, who used a moderate amount of tests, very little medical intervention, and was calming and relaxed.
If your doctor has you worried about not seeing a sac at 4 weeks, question that doctor's wisdom.