The result is adding 2 monsters or even 4 monsters to a combat is not going to have an appreciable effect. (4 monsters * 30 seconds * 6 rounds = 12 minutes worst case). Another mitigating factor to using more weaker monsters is that unless the party is monumentally tactically incompetent or seriously poor at generating damage these weaker monsters will go down pretty quickly.
You forget to take into account increased time for player response to 4 extra monsters and potential increased table talk. If there is just one foe in front of you, then it's pretty easy. Attack that foe. 2 foes, the decision making is a tiny bit tougher. Should I shift? Should I attack with a multi-foe attack? If I attack one, will the other get past me to the Wizard?
Now, let's take into account your concept of adding 4 monsters. 9 monsters instead of 5. That type of encounter can be a lot more challenging for the players cause they have to take a lot of variables into account.
It's not just the DM.
And, even the DM has to take more variables into account as well.
Going to KD's example the 27 HP level 1 monsters die 2x quicker than the 45 HP level 3 monsters. This means in your typical 6 round encounter these 7 starting monsters will be dying about 1 per round vs the 4 monsters that die roughly 1 every 2 rounds. This means you'll have only 4 monsters left on round 3 where with the higher level monsters you'll have 2-3 monsters left.
This is not quite true.
7 monsters will often be spread across an entire party. There is no way for the Defender to lock them all up.
Not all 5 PCs will be able to attack a single NPC and focus fire to the extent that the damage all goes on foe 1 first, on foe 2 second, on foe 3 third, etc. Some of the damage will be spread out across multiple foes and gradually wound foes. The PCs will be lucky to take out one foe in round one. Typically, the first of the 7 will drop in round 2, sometimes even in round 3. But round 2 on average unless the group has 2 Strikers or something.
In a 7 foe encounter, what typically happens is that one foe drops in round 2 due to 2 rounds of semi-focused fire, another drops in round 3, 2 drop in round 4, 1 drops in round 5 and 2 drop in round 6. And each round a foe drops, the focus fire on the NPCs in general increases slightly because a PC might free up in order to shift focus.
In a 4 foe encounter, what typically happens is that one foe drops in round 3 due to 3 rounds of focused fire, another drops in round 4, the third in round 5 and the last in round 6 as more and more PCs free up each round to increase focus fire on foes that are already wounded.
The monsters in the 7 foe case get 7 average attacks in round 1, 6.5 in round 2, 5.5 in round 3, 4 in round 4, 2.5 in round 5, and 1 in round 6 for a total of about 26.5 attacks (this assumes that the monster only gets its attack in the round it dies 50% of the time).
The monsters in the 4 foe case get 4 average attacks in round 1, 4 in round 2, 3.5 in round 3, 2.5 in round 4, 1.5 in round 5, and 0.5 in round 6 for a total of about 16 attacks.
Same number of monster hit points, but 60%+ more attacks.
Its true that the 7 monsters will have better action economy on their side as well and will probably dish out a bit more damage per round overall AT THE START, but the situation will shift later in the fight. I'd say the level 1 monsters will be a slightly greater challenge, depending on the makeup of the party,
It tends to shift pretty late in the fight. 7 instead of 4 is a huge difference, nearly double. The attacks are weaker, but there are a boatload more throughout most of the fight. If my numbers above are not unreasonable (and I don't think they are that far off the average mark), it becomes:
Round 1: 7 vs. 4
Round 2: 6.5 vs. 4
Round 3: 5.5 vs. 3.5
Round 4: 4 vs. 2.5
Round 5: 2.5 vs. 1.5
Round 6: 1 vs. 0.5
Sure, there will be exceptions, but in order for both encounters to extend out to a full 6 rounds, it's not going to be a situation where the PCs kill 3 NPCs in the 4 NPC scenario in round 6. Most of them will be killed earlier.
The action economy will favor the NPCs in the 7 foe case typically until at least round 3 (assuming a 6 round fight). In the 4 foe case, the PCs outnumber the NPCs and the action economy favors the PCs starting in round 1.
but KD also sounds like he's got a problem challenging his party when he talks about it being almost impossible to knock a character down by 5th level. I don't have that problem at all, combats are tense. The chance of characters going down is very real.
Can I play in your game? I never see this past the early levels, the PCs just have too many options. The only time this really happens is when the dice gods favor the DM instead of the players, specifically in the case where the NPCs have multiple area effect attacks and can whittle down multiple PCs at the same time. But with the amount of damage that monsters do? It takes a long time to knock a PC down when the PC Leader is just standing there stamping out the fires. And it gets worse as the levels get higher.
Note: The Cleric in our 16th level game has Berronar's Salve. She has not used it since level 10 or so which means that a non-Cleric PC has not gone unconscious since level 10 or so.
This is especially true now that a year and a half of splat books have come out. The monsters have not gotten any more optimized. The PC parties as a whole have increased in efficiency by probably 20% or more (due to increased synergies from everyone on the team). Nearly every PC does more damage than they did with just the core rules. Nearly every PC has more effective hindering NPC or helping PC options than they did with just the core rules.
The monsters haven't gotten that much better and DMs are being forced to increase numbers of monsters and/or levels of monsters and/or putting more hindering monsters into encounters in order to compensate (or more traps or whatever).
But playing in four 4E games (levels 4 through 16) at the moment, I do not see what you are seeing. It's tough to really challenge players without increasing the level of the foes. Look at Chzbro's example. An N+5 encounter that lasted 4 rounds. This illustrates pretty clearly (as does my play experience) that the higher the PCs get, the easier it becomes for them as they start acquiring more and more options. The monsters are just not designed to handle the non-linearly increasing synergies of the PCs. The only thing keeping the monsters standing are their mega-hit points. But pound for pound, higher level monsters average a lower percentage of the average PC hit points per attack, so higher level encounters become easier.
Sure, the first two or three rounds of an encounter are sometimes a bit scary as enemies focus fire on a few PCs, but by round four, the party is usually pretty much in control, even in higher level encounters. They just use more Dailies to accomplish it in the tougher fights.