My mares don't care much if the foals at 4 weeks go exploring as long as they can see them, and smell them. They will call to them if they get out of what the mare feels out of sight. I also have foals that will nurse from other mares, it seems they have babysitters in my herd, and community milk banks, the only mare the other foals won't nurse from is the alpha mare but her foal runs the roost and will nurse on who ever is convenient at the time. I don't know if it's possible for you to pony the foal when momma goes back to work on the trails, but it makes a a colt easier to break and be a good trail horse when they get older because of what they are exposed to on the trail. It takes a couple of times to get them used to the pony idea if the mare has never been ponied from and the foal isn't halter broke or broke to lead. I wouldn't do it down busy streets but in open country or nice trails.