A day after my missed period, when I was pregnant, it came out positive. Any other time I took a PT and it was negative it was bc I wasn't pregnant. wait another couple of days, if you still don't get AF then test again. The tests are usually right on. The only time I've seen a false negative test was bc it was before said missed period. So if you are indeed 4 days past your missed period, then test again when your 7 days passed.
It depends on how regular your cycle usually is and when you are taking them. Tests only pick up a minimum concentration of the pregnancy hormone, hCG, so if you're drinking a bunch to pee to test, you might have urine that is too dilute to detect at this point. That is why the most common suggestion is to wait to use first morning urine since it is usually the most concentrated of the day. There are rare cases in which women do not excrete substantial amounts of hCG in their urine and blood tests are required, but don't know the details since it's just something I have heard of.
It is also common to have delayed ovulation, especially if you have had a stressful month - it would be stress in the first half of your cycle, causing ovulation to be delayed (the luteal, post-ovulation, phase will typically not vary much). Unfortunately, the symptoms of pregnancy at such an early stage are very similar to PMS symptoms. I would suggest trying to put it out of your mind (easier said than done), waiting a few days and testing with FMU.