We have about 75 acres of land (most of which is forest) and 5 horses (had up to 8). But I have worked for farms that kept their horses in 15 meter round pens giving them 2 large flakes (from those giant bails) of hay. All the horses were healthy and worked regularily. Personally I feel if you have horses you should have a confinment large enough that they can get a good run by themselves.
If your having grass issues (as we did during one ridiculously hot summer) fence off a section of your field (in half usually). And alternate from side to side. But by the sounds of it you have two large horses and plan on getting 2 more large horses. I recomemmed saving up for lots of hay to keep up an adequate diet. If you can do that you should be perfectly fine.
A Dog looks up to a man,
A cat looks down on a man,
But a patient horse looks a man in the eye and sees him as an equal