From a 4th Edition perspective, none of this is interesting.
You're talking about stuff that happens between the encounters, where the rules are largely silent. Role-play, have fun, whatever -- but don't get stuck on how to handle it with rules.
4th Edition PCs are heroes. They don't have to worry about where to pee. They don't have to worry about how far to space themselves while traveling overland.
If you're suggesting ways to separate the party so that some of its members don't get to participate in a combat encounter, this is particularly not interesting--especially to the players who watch helplessly from the sidelines.
Let them travel how they like and get them to the next encounter quickly.
My personal hiking experience suggests that you can be hundreds of feet from one another and still have line of sight, hear them, or at least have a good idea where they are. This depends on tree density, brush density, and terrain.
Old forest can be surprisingly sparse under the trees, where the upper leaves block light and kill ground cover. Dried leaves and twigs produce an unmistakable crunching sound that gives you away for surprising distances. Young forest has more small trees and bushes.
Some of the densest wilderness I've been in was wet scrub, with six-foot-high bushes and grasses. The ground is soft but not necessarily mucky and the grasses camouflage you well. You could easily lose your friends in that.
People who know far better than I
Take a look at some of these links, which deal with squad tactics for modern soldiers. Some of these formations separate groups of soldiers by 10-50 meters.
If PCs are traveling overland and not expecting constant contact with the enemy, then they will probably spread out to around 10 meters between PCs (or about 5 squares), as visibility permits. In jungle or other extremely difficult terrain, PCs might have to go single file, but you'd still put reasonable distance between them.
Make it a Skill Challenge
4E already has a way to handle the stuff between encounters: skill challenges. Don't create a new subsystem; use the one the game already has.
I assume there's a reason they want to stick together. Determine if they manage to get where they need to go and maintain group cohesion via the skill challenge rules. If they fail, they get separated. Anyone who fails a roll in the skill challenge can end up separated from the group by N squares during the vital encounter.
The skill challenge probably has Nature and Stealth as primary skills. A player might make a good case for using Perception, Athletics, and Endurance as secondary skills. Insight or Diplomacy might help draw players back to the group fold.
All this leads into a wilderness encounter of some sort. Success at the skill challenge means getting to place characters in a reasonable place on the map. Failure means one or more characters are separated from the group, possibly to their great disadvantage. Perhaps they get ambushed and overwhelmed. Perhaps it takes them one or more rounds to catch up to the rest of the group.