Four acres is almost too small. IF the four acres is all in lush grass, they aren't sharing the paddock with a couple of horses or something and you soil test and fertilize regularly; I would try two pairs and a bull (if you can't AI). AI would save you considerable cost though given your scarcity of land. Those of us with 50 cows or more don't like to buy supplemental feed a lot; but most of us don't have your stocking rate either so I would keep a couple of feed sacks handy for when the cows get ahead of the grass during winter, the hottest part of the summer, or a drought in any time of the year. Commercial prices are so high right now and you are only buying TWO pairs so I would pay more and buy registered cows. YOu probably are NOT going to make money on two head most years either way so I would rather have cows I can brag about living next to my house instead of somebody else's culls and you always have the potential to sell your TWO calfs a year for a premium in the classifieds. I would just have fun with my cows and my kids and enjoy my life.