I am on Day 4 of my placebo pills but I have only had light brown discharge. Why is this happening?

Since you already had some withdrawal bleeding this month from when you missed a pill your period will be very light during this inactive week because you already shed some of the lining then. It hasnt had time to build up a full lining yet so you may just get spotting this time. It should return to normal next month. Anytime you get "breakthrough" bleeding, you can expect that following period to be lighter. This is just common sense when you think about why a period occurs. Each month a lining inside the uterus builds to receive a fertilized egg and when one doesnt occur, the unneeded lining is shed as our "period". Since one doesnt ovulate while on the Pill, we have a week without hormones to shed this lining since the usual hormonal triggers are changed. It stands to reason if you have breakthrough bleeding, you are not going to have a full normal period a week or two later. This is normal and nothing to worry about.

No, that was withdrawal bleeding from the hormone level dropping, not a period. You've still got 3 days left of inactive pills, you're not going to start exactly the same day all the time. When you're on the pill, the lining of the uterus is decreased so there is less to bleed, so periods are lighter. Its nothing to worry about.