There are three primary differences with player mortality in 4th Edition when compared to 3rd Edition.
1) Low level characters are a great deal more durable.
2) Within combat there is a great deal more healing and temp hp available to the players.
3) Damage does not carry over as directly from one encounter to the next.
The first element is pretty much a wash by the time you hit about 5th to 7th level, but it does have a huge affect on the initial adventures.
For the second element, I would say it is a bit harder for me to decide how much that affect scales. At low levels, it is huge. Assuming a single leader, you have 1 healing surge per player as a given, plus 2 more from the leader, and probably a few more lingering around from other abilities. A high level cleric in D&D can do a great deal of healing, but with 4th edition you can start a round with every player either bloodied or dying, and end the round with everyone standing with some significant amount of HP restored. Against a solo creature, that is huge.
The third element is the most telling though. If you end an encounter with everyone between 2 and 7 hp, you can take a short rest, and burn 4 healing surges each and suddenly everyone is back in top form.
I will still disagree that there is not much risk of death in 4th Edition. The games I have played in have actually had enough close calls and had enough people in danger of death that I cannot claim to see the lack of danger in games I have played. What I will say is that how and when that risk shows up is different.
Within a single combat, you need to have monsters capable of consistently hitting the players, and either have the fight last a while. If you are using creatures a few levels below the PC's, or only a few at equal level, you simply will not inflict enough damage. If the PC's can clear all the monsters fast, you have the same problem. But it is absolutely possible to push the players to the brink of death and into death saves with level approprite combatants. Encounters built around single opponents wont do it, but a good mix of Minions and typical opponents can do it.
If you want to do the job across multiple encounters, then it is not about the HP you burn up, but how many healing surges you can force them to use, and whether or not you can justify hitting them before they can take a short or long rest.
Without mechanical adjustments, here are what I will suggest you try.
1) Do not get too hung up on the Defender. Getting marked is inconvenient, but you are seriously better off having a bad guy eat an opportunity attack and the -2 penalty as long as you can get around the Defender and go after the back ranks.
2) Work the situational bonus. If you do have to go after a defender, you need to dogpile the hell out of him with flanking, and hopefully have him dazed or marked or whatever else you can manage. When your creating an encounter look for potential combo's. If you have a Skirmisher who gets extra damage with combat advantage, then why not throw in a other combatants who can Daze or Stun someone.
3) Do some match making. If you have lots of minions, have them dogpile the striker. If you need to go after the defender in melee, that is where you put your Soldiers or Brutes. Put your artilery on their controllers. Put your skirmishers on someone not wearing plate armour. And put a melee guy next to the Controllers. Use your push / pull / slide to put the rogue where he cannot get combat advantage via a flank.
In general, the DM cannot count on dropping a target in 1 or 2 rounds from a single combatant. The DM will need to be able to sustain pressure for a bit longer then you would under 3rd edition.
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