I have a 21 day cycle and am 4 days late. I spotted some. This is unusual for me. I have an iud. Should I be concerned about an ectopic pregnancy?

Yes. You should be concerned but not alarmed. You should first take a pregnancy test to see if you are pregnant and if so should get an ultrasound with your doctor to locate the pregnancy.
Aware, not concerned. Do a home test today and consult a doctor soon if positive. Iud does not increase risk of ectopic pregnancy over baseline, because they are so effective for birth control. Getting pregnant at all with an iud is uncommon. If you got pregnant with an iud, the ectopic risk is elevated.