1 point : 300 damage over 12 sec.
2 points: 498 damage over 12 sec.
3 points: 696 damage over 12 sec.
4 points: 894 damage over 12 sec.
5 points: 1092 damage over 12 sec.
Taken from Wowhead.
Using the formula you just pasted, I get:
AP 4CP Damage Per Tick (4) 5CP Damage Per Tick (5)
2000 1964.82 327.47 2247.96 374.66
3000 2308.02 384.67 2591.16 431.86
4000 2651.22 441.87 2934.36 489.06
5000 2994.42 499.07 3277.56 546.26
That's doing:
(894+(AP*0.24))*1.3*1.1 and then the result/6 for each tick.
The multipliers are for mangle and naturalist, and the 5 cp formula uses 1092 instead of 894. I'm definitely missing something, or it doesn't scale like that (seeing as I normally have around 5k ap with might/kings/enh shaman and get ticks of 650+)
At 5000 AP, DpE is ~100 for 4CP, and 109 for 5CP. At 1000 AP its 54 and 63.5 respectively using this, although I know the numbers are wrong so it doesn't count for much.
Just playing around with it, changing the factor for 5CP to 0.3 gives a value of 617/tick for 5CP, which looks much closer to the truth (but is still some way short as far as I can tell).
In any case, as far as I can tell it won't be worth using 4cp rips ever from a DPE standpoint unless the scaling changes remarkably.