Cervix seems very sensitive during early pregnancy (4 wks). Brown spotting and bleeding after sex. Is a test of progesterone level needed?

No. The cervical blood vessels can be very near the surface. They are more engorged with blood and become more fragile due to the pregnancy and increased estrogen. The cervix always seems more "touchy" and cramps during pregnancy. All of this is normal. But you should be checked to make sure there is no other cause for your bleeding.