Do you know how old the trees are? 4-6 foot sounds like about 3 years old, but it depends on the rootstock. A standard or semidwarf rootstock can let a 2 year old tree get pretty tall. How far apart, again, depends on rootstock and also depends on how much pruning and training you want to do. If you are eager to practice a lot of pruning and training, and you don't want very many fruits per tree or have a small space, you can plant them as close as a couple of feet apart. If you don't want to prune more than once a year and have plenty of space for them to spread out, then it depends on the rootstock. Standard trees have to be, hmm, about 30-40 feet apart till all is said and done. I have my dwarf trees spaced about 10-12 feet apart.
Trees really should not be allowed to bear fruit, or not allowed to bear more than a few fruits (I know, it's hard to get your first apples and have to rip them off--just a couple are OK to leave ripen) until they're about 5 years old. You want them to establish a good root system first. Their first real crop at the 5 year point, you should thin off 1/2 - 2/3 of the fruits also: they grow in clusters, so thin them down to one little green fruit per cluster. If you don't thin them out, you'll get a bunch of puny little things that are quite disappointing, with oogy spots where the fruits touch each other, and it puts more wear and tear on the tree than is really healthy. Use those green fruits for pectin for jam, or give 'em to the critters.
