I find it very hard to believe that you would have gained four pounds of fat durring four days because that would have ment you injested 14,000 EXTRA callories durring that time. Durring your PMS and your period, women bloat and retain water. If you've been exersizing and eating properly I doubt you gained four pounds of fat. Muscle weighs a great deal more than fat does as well so if you've been training hard you could very well have put on a little bit of muscle but not four pounds of it. My guess is that it's simple that time of the month that's causing it. Make sure you drink lots of WATER. But Im sure your fine!
I'm sure you will trim right back down after your period ends. You are doing all the necessary exercise to keep yourself fit.
Weight fluctuates all the time -- why are you checking it so obsessively would be my question for you to ask yourself.