You can also cut the pasture in half. Put them on one have for a month or so and then put them on the other. This was each half gets a break and never completely eaten down. If I am not mistaken the general rule of thumb is one acre per horse - so I think you should be fine. I have roughly 2.5 acres that I am going to fence for my two. It will be cut in half so I can monitor how low they eat it. If I see that they need more area then I will open both sides up to them.
The other thing is if you cut it in two to keep them off one side of the pasture, if you see that they are eating it down too far, stick a bale of hay in each side for them to have as well.
Just my opinion - hope it helps.