(sorry for the color scale which makes it look like something from the 80s)
The data is calculated based on the assumption that you have 900 base atk, and 900, 1200 or 1500 atk from 12356 slots. I picked these values to show you the influence of rune atk on the result.
(Comment #1: 900 base atk with 900 rune atk is the same as 800 base atk with 800 rune atk.)
X ? CR with the 12356 slots
Y ? CD with 12356 slots
(Comments #2: Despite the optimal region for atk% on the 4th slot being eaten away when your rune based atk becomes high enough, the optimal regions for CR and CD remains relatively fixed. So normally you know what you should use on your 4th slot, even if your base/rune atk is different from what I have shown)
EDIT: Clarifications The X values is the CR that you achieved without the 58% on the 4th slot.
The Y values is the CD that you achieved without the 80% on the 4th slot.
Basically you equip 12356 slots and check the CR and CD you have, then you read the graph to decide if you should use atk% CD or CR on the 4th slot
EDIT2: Another comment, you don?t really need to know exactly where lies the boundary between using CR and CD, if your runes put you close to the boundary, it basically means that there will be negligible difference between using CR and CD on the 4th slot. Just pick the better runes and no worries xD.
EDIT3: Whatever your monster is, normally if you have two atk% on both 2and6 slots, you should be looking at image #3 or #2, if you have only one atk% on these slots, you should be looking at image #1.
EDIT4: You don?t really need to read this because it (the contour maps) is not very intuitive, unless you really want to know how much difference will your choice make, I?ll just show an example of atk=900+900
For example, if you have 25CR and 30CD, (25CR and 80CD on graph #3), the average damage difference between using CD and CR is about 20%. (yellow line)
If you have very low CR (20%) and very high CD (70 CD, 40 of which can come from the rage set), and you insisted on using CD and not CR on the 4th slot, the damage you lost might be as high as 40% (green line)