Inspired by this answer to a question:
How could you adapt the Vitality and Wound Points system from Unearthed Arcana to the 4th ed rules.
What major changes would you need to make?
Quoth the SRD:
"Vitality points are a measure of a character’s ability to turn a direct hit into a graze or a glancing blow with no serious consequences. Like hit points in the standard d20 rules, vitality points go up with level, giving high-level characters more ability to shrug off attacks. Most types of damage reduce vitality points.
Characters gain vitality points as they gain levels. Just as with hit points in the standard d20 rules, at each level a character rolls a vitality die and adds his Constitution modifier, adding the total to his vitality point total. (And, just as with hit points, a character always gains at least 1 vitality point per level, regardless of his roll or Constitution modifier.) A 1st-level character gets the maximum vitality die result rather than rolling, as shown on Table 4-6 below."
Wound points measure how much true physical damage a character can withstand. Damage reduces wound points only after all vitality points are gone, or when a character is struck by a critical hit. A character has a number of wound points equal to her current Constitution score.
In 4e, a monster will do Level+8 damage on a hit which roughly works out to 1 healing surge of damage. Most parties have the ability to spend Players+2-4 surges per fight, depending on the leader build and how much self-healing the defender is prepared to do.
The real problem with this mod is that it will slow down the healing time significantly. It is not particularly useful to map surges to wound points, as any monster will burn through the surges as if they were not even there. Instead, we care about surges as a count of actual lethal blows someone can take. Someone's surges are a function of their class and constitution, and so can represent this to some degree.
Presuming you're not using heightened damage systems (I would be terrified of using this system with a fourthcore approach or a "level 1 scaled damage" approach) A character who is out of HP (vitality points) can take damage directly to her healing surges. For every Healing surge value (1/4 of her HP) she takes after being reduced to 0 hp, she loses 1 healing surge (read: wound damage). Every death saving throw she fails also causes 1 wound damage, though being reduced to 0 hp only induces the dazed condition on the character, instead of unconscious. Having someone's healing surge count hit 0 kills them. Instantly.
Healing surges lost in this way recover as per the wound recovery rules, not simply on an extended rest. Surges spent for "vitality point" healing are recovered normally.
What this should end up doing is providing characters with a temptation to "keep on fighting" but with reduces subsequent capacity on future days of fighting. For added "realism" reduce the amount of healing that may be gained from surge-spending. As this realism works out to "we spend more days resting" it's not particularly interesting from a game design point of view.
Still, by having surges lost on death saving throws, and making the consequence of death saving throws not simply something that go away on short rest, players (especially defenders) will feel far less sanguine about playing yo-yos during fights. (low level defenders often spend more than a quarter of any given fight unconscious. It's tremendous fun.) Characters can still die from being beaten up upon when they're "down" but so long as a dying character doesn't thrash too much, few goblins will take the time to "put the boot in."
The ultimate consequence of this change will be to make stabilization of dying characters far more of a priority and to enable far more "heroic" last stands by people willing to sacrifice their healing surges to tank that last round or two with the big bad evil guy without imposing too many external costs on the system.
With regards to monsters? Ignore this system entirely. Monsters should have HP appropriate to your game, depending what scaling you're using and what version of the Monster Manual's algorithms you choose.