Hi, the dark blood spotting for two days after 4 days of missed periods could be either the symptoms of a pregnancy
or an abnormal cycle. Generally a delay of upto 7 days beyond the expected date of periods is considered normal and it has been a 4 day delay yet. However if your cycles have always been regular and you have had intercourse wlthout contraception
in this cycle it is more likely to be due to a pregnancy. I would suggest you get a serum beta hcg
test done. It's more sensitive than a urine pregnancy test and if pregnancy is confirmed, will help in ascertaining whether the spotting is just a normal implantation
bleeding or due to any abnormality in the pregnancy. However if the beta hcg levels are normal you can take hormonal pills like Regesterone
to induce withdrawal bleeding and regularise your cycles.