Before build a 4 players cab, i would ask myself a few questions:
1.- Will I play 4 players games often? Like 2-3 days/month at least?
2.- Will my Control Panel be comfortable with 4 complete joystics + buttons?
3- Do i know how many 4 player games are there? And how much of that games do i like to play?
I asked myself this questions when building my cab, and i finally ended building a 2 Players cab (*)
(*) I set it up with the possibility of playing 4 Player games with 2 Joystics + 2 Xbox 360 wireless controllers or with 4 Xbox 360 wireless controllers.
I can also play 6 player games with 2 Joystics + 4 Xbox 360 wireless controllers.