I'm curious, is this a fourth edition problem? Because I'm not aware of such an issue in other rule sets (and speak from having run all of them save 4 and 3.5, though I've played enough 3.5 to not recognize this). Not trying to derail, I am just baffled at the existence of such a thing.
(Conversely, you might well understand now why I misunderstood the thrust of the O.P., if in fact I did.)
I accidentally introduced a house rule in 4e giving charge +2 to speed, which made retreating harder. Whoops.
But I saw retreating problems in 3.x, and would carry through to 4e without accidental house rules.
3e withdraw:
Withdrawing from melee combat is a full-round action. When you withdraw, you can move up to double your speed. The square you start out in is not considered threatened by any opponent you can see, and therefore visible enemies do not get attacks of opportunity against you when you move from that square. (Invisible enemies still get attacks of opportunity against you, and you can’t withdraw from combat if you’re blinded.) You can’t take a 5-foot step during the same round in which you withdraw.
If, during the process of withdrawing, you move out of a threatened square (other than the one you started in), enemies get attacks of opportunity as normal.
You may not withdraw using a form of movement for which you don’t have a listed speed.
Note that despite the name of this action, you don’t actually have to leave combat entirely.
And charging
Charging is a special full-round action that allows you to move up to twice your speed and attack during the action. However, it carries tight restrictions on how you can move.
Movement During a Charge
You must move before your attack, not after. You must move at least 10 feet (2 squares) and may move up to double your speed directly toward the designated opponent.
You must have a clear path toward the opponent, and nothing can hinder your movement (such as difficult terrain or obstacles).
In an open field, withdrawing is lethal. You double move, your opponent charges you and hits you. You keep withdrawing, they keep charging.
Obviously there's ways around this, but other than terrain most of these methods take actions. Have the wizard cast Web? That's a standard action. Dropping caltrops? That's an action. (Also, caltrops are pathetic in 3.x). Throw a smoke grenade? Watch the game grind to a halt as the DM and players look up the concealment rules. Withdraw through difficult terrain? Your opponent can't charge, but they can follow into the terrain, or go around, possibly even meeting you on the other side! Cast Invisibility? Works on only one PC, and since the PCs are losing anyway, they aren't likely to be happy if a PC sneaks off by themself.
And, of course, opponents might not want to follow you into melee. They might shoot you. If they've got longbows or shortbows, rather than crossbows, reloading doesn't take actions. They can take a move action and shoot, and their attack range if far greater than your withdraw speed. (A longbow has a range increment of 80 to 110 feet, assuming no Far Shot, which is greater than your withdraw range, and the archer probably has the same speed as anyone but a monk, since they don't wear heavy armor; that could mean it takes three rounds before the archer takes any attack penalties for range; if the archer is "specialized" and not just a melee fighter using a bow as a backup weapon, Far Shot is common.) Even with all the penalties as you get further and further away, the pursuers are getting free attacks. This is just "on paper".
In practice, PCs won't retreat until someone is injured, and now you've got a cleric trying to manage the impossible task of running away from opponents, moving toward a wounded comrade (there goes their move action) and then casting a heal spell (there goes their standard action), or casting Mass Cure X Wounds, which doesn't heal a whole lot. (4e clerics can heal as minor actions, but can only do this a few times. Ranges are shorter, but this just gives incentives to the winning NPCs to keep pursuing.)
It works better in a dungeon, because you can just go around a corner, and an opponent can't charge you. In this case, your opponent (if they have the same speed) can catch up. (If they've got reach, withdrawing isn't going to work, but plenty of opponents don't have reach.) They can literally follow you for hours, as withdrawing doesn't actually give you any advantage other than making your first step a 5 foot step.
All of this assumes there's no NPC spellcasters. It's hard to run away if they can do anything to the ground, like Spike Growth, or Soften Earth and Stone, or Entangle (a 1st-level spell), or can cast spells like Glitterdust (I think you move at half speed if you're blinded in 3.x)... The NPCs only need to focus on one PC, since the PCs will not allow a friend to die, and will fight to the death to defend them rather than run away.
4e withdraw: I can't even find it, though I swore it existed somewhere in the combat section. I probably used 3.x rules accidentally. Maybe that's the reason running away is harder.
Ranged attacks are different. A longbow has a range of 20/40, which is less than the range in 3.5, but a longer
accurate range. (You can't hit someone 5 range bands away for a -8 or -10 penalty.) Still, the range of a longbow is greater than a running PC's speed.
Quite a lot of opponents have effects that can slow or trip you (common among soldiers, the latter among brutes, and both among controllers).
I think withdraw is a generally poorly understood part of the rules. It's not
hard to understand, though, but you use it less often than making attacks.
The biggest rule handicap to running away in D&D is the way the initiative system works and opportunity attacks, I believe. You move your full move, and then your opponent gets their full move... In real life, if I'm chasing someone with a nerf sword, I pretty much have to stop moving (or stop running and start walking) after hitting someone with the sword (or missing). I think in real life you have to "commit" once you've attacked. If you fail to "down" your opponent, it's over. They've gotten away. However, in D&D, and probably most games that allow opportunity attacks, you can follow and get opportunity attacks forever. It doesn't help that players seem to fear opportunity attacks, going to
great lengths to avoid them. They know running away could mean provoking opportunity attacks (sometimes without a chance to strike back), and so they just ... won't.