If it is about power creep, then its likely MM4 is a preemptive strike. Then when you get owned by this stuff, you will hand over the weekly paycheck to equip your PCs with the latest and greatest.
Most of the monsters are okay, if you are ready to deal with them (as both PC and DM). Like when we first fought those Drowned from MM3. Two of those smashed our group of something like 14th level chars, remember? But then we fought one in your low-magic world at lvl 9, and it wasn't too bad...
Another example, check out the sailsnake. As far as hp, att/dmg rolls/etc its probably about the same as a varag, but its CR 2. Okay, not too bad, you do get more XP for this... oh yeah, forgot to mention its breath weapon, 20-foot cone of blinding venom. Get this, blind for 1d4 rounds, DC 13 Fort for half. So basically it could blind the whole party, even if they made the save! The most balanced stuff in it are the advaned MM1 monsters (and they are still pretty optimized chars, ie CR 4 orc bbn with +13 att 1d12+10/x3 dmg, plus a potion of shield of faith +4. So when he drinks it in front of you, not only does he get a +4 AC, you know that you just lost out on some of the loot.)
What I am getting at, should your PC ever come across a bluespawn godslayer or a pack of bluespawn stormlizards, run for the hills. There is no XP, there is no loot, but there is a quick death. These guys are even tougher than the greenspawn razorfiend (aka harrowblade), and we've seen a harrowblade shred a lvl 8 dwarven defender before he knew what was even going on (harrowblades are listed as CR 7).