You'll need the IPAQ4 for sure. Will it be enough? Well, its got
"slots" for 4 joysticks(U/D/L/R x 4) with I think 6 or 8 buttons each, plus all of the necessary buttons for mame (player start 1,2,3,4) and player coin 1,2,3,4.
That ought to cover it. Since I can't think of any game that is 4 player with 6 buttons.
Your spinner/trackball issues are more complex, but only because you have more options.
You can use OscarControl spinners and use his USB interfvace, or wire them through an OptiPac (which has inputs for 4 spinners or 2 trackballs....(2 x axis, and 2 y axis), or buy the happs thing I guess with their interface.
Just remember in order to have more than 1 trackball or more than 1 spinner you have to use Mame Analog and not regular mame.
You can go with a Happs or Wico trackball, using the optipac or their own interfaces...or you can get a PC trackball (serial or USB or PS2 mouseport) and just use that.....cheaper, but you lose some of that arcade feel I guess.
Good Luck. There are lots of ways to skin this particular cat.
Dont forget about rotating joysticks for Ikari Warriors and such!!
