Scott,
If you know they are four miles away, I'd think you have at least a few in your area. They tend to wonder around allot and that's not very far.
In my experience, they group up and stay in an area for awhile, then for whatever reason, they will move on. Sometimes my corn will rot on the ground for a month or two without any activity. Other times, I'm overrun with hogs.
I've read stories, and they are just that, stories witout anything to back them up, that in ideal conditions, a sow can have three litters a year. From what I've seen with sows and piglets, I think two is about it, but that's still allot of new animals coming into the area every year. Two litters of six animals will make a huge difference in the population in just a few years!!! Especially when there are dozens and dozens of sows in the area.
Having them arond is really fun, but if I had a choice, I'd rather not have them and hopefully have more deer. We rarely see any deer here and I think the hog population has allot to do with it. Of course, it's allot of fun to shoot them, eat them, take pictures of them and just watch them. Lately I've started trying to see how close I can get to them without scaring them off. Last week I was about 50 yards away, but it was too dark for pictures. It's fun when you can hear them really good!!!!
Eddie