Having thought about this for a while, I think that the OP's model probably won't work. Combats will be too swingy with considerably higher level foes taken out too easy and considerably lower level foes taking out the PCs too easily.
But what might work is a hybrid model.
Instead of monsters getting +1 to hit and defenses every single level like 4E and +1 to hit and defenses every 5 levels like in this model, what if they only got +1 to hit and defenses every other level?
Using first level 4E PCs and monsters as a baseline, this would mean that a foe 10 levels higher than the PCs would only be +5 to hit and +5 defenses compared to the PCs (plus the ton of extra hit points and damage). PCs could then get a +1 to hit and defenses every 3 levels.
30th level monsters would be +15 and defenses to hit over 1st level monsters (assuming the bonus comes in on even levels). 30th level PCs would be +10 to hit over 1st level PCs (assuming the bonus comes in on levels 3, 6, etc. for simplicity). The other +5 at level 30 comes from either inherent bonuses, or magical items (+1 inherent levels 5, 10, 15, etc.).
Note: on the concept of inherent bonuses vs. magic items, I think the plus to hit of both should be the same. But, I think that magic items should also get the bonus to damage that inherent bonuses should not.
As an example (with PCs gaining 1 points of damage per level, similar to what most PCs might gain in 4E). 30th level NPC vs 30th level PC:
PC has a normal non-magical weapon. PC is +15 to hit over first level and +29 damage over first level.
PC has a +2 magical weapon. PC is +15 to hit over first level and +31 damage over first level.
PC has a +4 magical weapon. PC is +15 to hit over first level and +33 damage over first level.
So, the magical weapons are still coveted for their increased damage (and other abilities), but there is no actual requirement to hand them out at all. The PC fighting with the normal non-magical weapon at level 30 still does some serious damage (say 38 points) compared to the PC at level 30 that has a +5 magic weapon (say 43 points). Give or take.
A 30th level PC could pick up a chair and fight with it and would still be doing some serious damage. This resolves one of the 4E issues without having the swinginess that the OP's system introduces.
From the current model, monsters change from +1 to hit and +1 defense once per level to every other level. PCs change from +1 to hit and +1 defense every other level plus magic plus ability score changes plus feats, to +1 to hit and +1 defense every third level plus magic/inherent. It drops ability score boosts, feat boosts, masterwork armor boosts, a plethora of other boosts out of the equation.
As a rough idea:
Code:
PC hit magic Monster hit
level to hit AC points damage damage to hit AC points damage
1 4 18 25 9 9 5 15 25 9
2 4 18 31 10 10 6 16 31 11
3 5 19 37 11 11 6 16 37 12
4 5 19 43 12 12 7 17 43 14
5 6 20 49 13 14 7 17 49 15
6 7 21 55 14 15 8 18 55 17
7 7 21 61 15 16 8 18 61 18
8 7 21 67 16 17 9 19 67 20
9 8 22 73 17 18 9 19 73 21
10 9 23 79 18 20 10 20 79 23
11 9 23 85 19 21 10 20 85 24
12 10 24 91 20 22 11 21 91 26
13 10 24 97 21 23 11 21 97 27
14 10 24 103 22 24 12 22 103 29
15 12 26 109 23 26 12 22 109 30
16 12 26 115 24 27 13 23 115 32
17 12 26 121 25 28 13 23 121 33
18 13 27 127 26 29 14 24 127 35
19 13 27 133 27 30 14 24 133 36
20 14 28 139 28 32 15 25 139 38
21 15 29 145 29 33 15 25 145 39
22 15 29 151 30 34 16 26 151 41
23 15 29 157 31 35 16 26 157 42
24 16 30 163 32 36 17 27 163 44
25 17 31 169 33 38 17 27 169 45
26 17 31 175 34 39 18 28 175 47
27 18 32 181 35 40 18 28 181 48
28 18 32 187 36 41 19 29 187 50
29 18 32 193 37 42 19 29 193 51
30 19 33 199 38 43 20 30 199 53
So, the magic damage column is the damage if the PC has gets a +1 increasing bonus item at levels 5, 10, 15, 20, and 25 (no inherent or magic bonus at level 30).
This isn't a perfect model. The damage column for the PCs and monsters are a rough idea based on the fact that there will be extra synergies at higher levels. For example, monsters with claw, claw, bite; or PCs with multiple attacks per round or AoE attacks, or striker damage. Auras, Ongoing damage, etc.
I sort of took that into account with the monster damage (which for any given attack would only probably be +1 damage per level like in the current 4E model), but would be more overall. I didn't do that for the PC damage column, just to give a rough idea of what a single PC attack might do (without bonus striker damage or without AoEs, etc.).
The PC's to hit and AC get a 2 jump at level 15. +1 for every 3rd level and +1 for inherent/magic bonus.
But, the rough idea is here. A 9th level PC fighting a 19th level NPC is +8 to hit AC 24. He needs a 16. Doable, but still extremely tough.
Course, this like any other model, only works if the game designers stick to it and don't start handing out offensive and defensive bonuses like candy via feats and class (paragon class, epic destiny) abilities like in 4E. Granted, part of the extra bonuses in 4E are to fix the math problem, but they really have to curb that tendency to have stackable bonuses scattered throughout the game system.