After much thought about combo 2/4 player control panels, I've come to a few realizations...
There are only a limited number of ways to lay out a control panel given the following assumptions...
1) player 1 is always to the left of player 2
2) player 3 is always to the left of player 4
If you number the positions of the player's joysticks from left to right, you can only have one of the following patterns:
a) 1234
b) 1324
c) 1342
d) 3124
e) 3142
f) 3412
Layouts 'a' and 'f' probably aren't any good because player 1 & 2 are crammed into the left or right half of the panel.
Layout 'c' is interesting because it puts the maximum distance between player 1 and 2. However, this would probably result in player 3 and 4 being too cramped in the middle.
Layout 'd' is nice because player 1 and 2 will be directly in front of the screen. However, with a small control panel, this may prove too cramped for everyday use.
That leaves two layouts, 'b' and 'e'. These are the layouts that I think most 4 player panel designers arrive at eventually (or something very close to them). While neither player 1 or 2 is perfectly centered on the screen they are separated by a player (3 or 4) in between them which gives a little extra elbow room on 2 player games.
(The images below taken from another post. I'm sorry that I cannot remember who originally posted them. If you see this, just let me know and I'll give you credit)
This is the
1324 layout:

The advantage here is that the joysticks are somewhat evenly spaced for maximum comfort in 4 player games. However, this layout isn't ideal for Smash TV or Robotron as the sticks are probably too far apart.
This is the
3142 layout:

The main advantage to this layout is that it is ideal for playing SmashTV or Robotron. The sticks can be spaced fairly close together as they are on a Smash TV machine. The obvious drawback is that the joysticks are a lot less evenly spaced and will be more cramped in a 4 player game. However, the button positioning might be better for 4 player games than the 1324 layout.
...
Krick