You can't do that with vacation. Pretty much anything that is considered "work," such as CQ, other sim events, ALPA duties, etc... will allow you to go below 4 days. You can only go less than 4 days at the beginning of the month IF you had work on the last day of the previous bid month that would make it a 4 day stretch. At the end of the month you can always put as little as 1 day of reserve because you could always bid reserve (or a trip) at the beginning of the next month that would make a 4 day block.
I highly recommended you get the John Bell reserve line simulator. I use it every single month I bid reserve (nearly every month) just to double check that what I'm asking for is legal. It's saved me plenty.
Reserve Line Simulator