I haven't built the 4-player panel (yet?), but one thing that I see in a lot of panels is that the joysticks aren't set up very well for the dual joystick games - Robotron, Karate Champ Vs, Munchmobile, Smash TV, Total Carnage... These games are some of the best, and they'd be kinda ruined if pairs of joysticks weren't aligned so that their x-axes were in the same line.
There would be two ways to do it right for these games. Either put all 4 joysticks along one line, or angle each pair of joysticks. Probably the first idea is a lot better, because you'd want to be in the center to play a game like Robotron. Also, it would probably be awkward to play a two player game at an angle.
I suppose that you would end up sacrificing some of the comfort for game like Gauntlet, however, so I guess you gotta build several cabinets to make every game completely perfect. For me, personally, getting 4 players around the machine at one time would probably be a rarity, so I would prioritize the 2 player dual joystick games over the 4-player comfort.
Heck, I haven't drawn out plans, so maybe the width problem you're talking about kinda kills this whole idea, I dunno. I have a 2 player panel setup now, and I'm thinking about making 2 1-player mini-panels that could be hooked onto the sides for when 4 players arrive.