I think a big consideration here ought to be aesthetics. There's little doubt that, assuming the mounting nuts inside the hull are firmly secured in place, two pedestals will keep the model from falling over in normal circumstances. But I've seen quite a few models of big, wide, twentieth-century warships mounted on four pedestals (arranged in a + configuration, with two on the keel fore and aft and two outboard amidships), and they looked fine. Better, in fact than two would have looked. A big, squat battleship mounted on only two pedestals may look like it's nervous about falling over - even if it really isn't.
Ed's made an excellent point (which is applicable to virtually all static ship models): if at all possible, work out the mounting system is at the beginning of the project. Drilling holes and securing nuts in a styrene hull is ludicrously easy if you can work from the inside. (Another approach, particularly applicable in the case of a ship with a flat bottom, is to epoxy a piece of wood inside the hull and use wood screws through the pedestals. I've known nuts to come loose because, years later, I had some reason to take the model off the pedestals and tightened the bolts a little too much when I put it back. A good, big piece of wood is much less likely to do that.)
Another tip that's worked for me: if you do epoxy (or superglue) nuts to the inside of the hull, use plastic cement to stick some nice, thick chunks of plastic to the hull bottom alongside the nuts. The plastic cement is less likely to come loose than the epoxy, and will help keep the nuts from turning. Better yet - cut a hexagonal hole to fit the nut in a thick piece of plastic sheet, and glue that inside the hull bottom.
As Ed noted, if you're in the awkward position of mounting the pedestals after the hull's been assembled, you've got an additional reason to use four pedestals: the fastenings for them will be less crucial. And (assuming all four pedestals are exactly the same height) you don't have to worry about the ship listing to port or starboard.
I used to have reservations about mounting models on lamp finials, because they tended to look so much like...well, like lamp finials. I've noticed recently, though, that the lamp companies have started producing quite a variety of them. At our last model club meeting a member brought in a beautiful, plank-on-frame model of a sailing revenue cutter, enclosed in an equally beautiful walnut case that he'd built for it. The model sat on a pair of exquisite, polished brass pedestals that, to my eye, were just the right height and in just the right locations along the keel to set off the model perfectly. I practically dropped my teeth when the builder explained that they were lamp finials with grooves filed in their tops.