I would add a 5th (or more, if you feel the need to break it up): the DM. This includes: how challenging the encounters are (XP budget); whether he/she tweaks monsters to make them tougher or creates encounters with nasty synergies and/or terrain (so, non XP budget toughness); how tactical the GM is; how nasty the DM is (non-fudging, letting players spot dangers, coup-de-gras, focus fire); and length of the encounter day.
Now, my PCs are pretty week on your criteria, but I'm pretty weak on my DM criteria... so with 2.5 leaders in 4 PCs, its been pretty easy for them. That said, with MM3 I've kicked things up a notch, and have come pretty close to running them out of surges in 4 N+2 encounters, something I have never done before.
The reason I didn't add DM nastiness to the equation is because I think that I am a DM who is pretty tough and the PCs still do not fall unconscious too ofent and never die.
For example, I don't fudge dice rolls (I roll in front of the players), I don't fudge which PC to attack, and two thirds or more of my encounter are N+2 or higher. Most of my encounters have some type of hindering or damaging terrain. And I play my monsters as dangerous as I can make them (for example, I used a Hydra on Sunday that was marked by the Fighter, so would attack the Fighter with one head and some other easier to hit PC with the other heads, I didn't do this in the first melee round for the Hydra, but once the Hydra couldn't really hit the Fighter, it went off in search of easier prey).
I don't use PC Passive Perception for hidden stuff (players have to explicitly look around). My theory is that if it is hidden, then one cannot spot it unless explicitly looking for it. And I don't give out Monster Knowledge checks for free (mostly because I don't think to do so and in most fights, my players don't remember to do so either).
Overall from a push over to rat bastard DM rating of 1 to 10, I'd give myself a 7 or 8. The only nice thing I tend to do that I can think of is that I don't typically use a Coup De Grace on a downed PC (I have on occasion, but it requires specific circumstances like a reoccuring villain or something), then again, the PCs are rarely down.
The problem is one of splat books IMO. As more time goes on, the PCs get more and more capable and their synergies get more and more powerful.
The monsters stay the same. Sure, the DM can go out of his way to find specific monster synergies that work well together, but that's harder to do day in and day out than it is for the players to find a good combo once and retrain into it.