Took only 3/4 pill free days. withdrawal bleed didn't show, started new pack, took 4 pills, stopped for 4 days, got an unusually light bleed. cause ?

Vaginal bleeding: The bleeding you had likely was withdrawal bleeding because you stopped the pill for 4 days . But would check HCG .

Breakthrough bleed: Breakthrough bleeding (bleeding at the wrong time) is very common on birth control. It happens in about 1/3 of users. Its no big deal and usually self corrects over time. If you are not satisfied or it continues to happen then consider using a different birth control method. It is a common side effect and does not mean something is wrong or that the pill is not working.