Split systems count as 0.5 of each system. You cannot have more than 2 systems at any range, but all of the following are legal:
2 dedicated systems: 2 Direct Fire systems =
1 dedicated and 1 split: 1.5 Direct Fire systems, 0.5 Arty systems =

/
1 dedicated and 2 split: 2 (1+0.5+0.5) Direct Fire systems + 1 (0.5+0.5) Arty system =

+

/

+

/
4 split systems: 2 (0.5×4) Direct Fire systems + 2 (0.5×4) Arty systems:

/

+

/

+

/

+

/

- You only get the d8 with
two complete systems dedicated to a range.
- 4d6 is possible and legal in some of these configurations.
- Statistically, 2d6+d8 is better than 3d6.
- Statistically, it's arguable that 2d6+d8 is still better than 4d6. (
http://anydice.com/program/8967)
- If you dedicate enough systems to get to 4d6, your frame will be very weak in other areas such as defense, movement, and spotting. It's generally a bad idea.
Theoretically, you could use an above 4d6 frame configuration to do a one-time roll of

. You'd probably do very well on that roll, but it still won't be more than 8—and an 8 is possible every turn with a

frame.