the problem you will run into, is that there aren't many sticks that are easily switchable betwen 4 and 8 way. the only two that i know of are the magstik plus, and the omnistick prodigy...the omnistick has a mounting plate that goes on top of the panel, so it isn't going to just drop into your x-arcade...and the magstik i believe has a different size mounting plate, so it won't just bolt right in either.
the ultimarc J-stik comes in a red balltop, and has the same mounting holes as the x-arcade sticks, and it is switchable, but you have to switch it from below the panel.