The ST3 isn't that much of a simpler bike, it's newer though. You could buy an 07, which will bring you right into the more modern updated look for that bike, which is nice if you can find one. However, if you can't find one and your stuck between an old ST3 or ST2 vs ST4, I'd go with the ST4 every day of the week. Its a heavy bike and having that bigger motor is a good thing in my view. I've ridden the ST2's and ST4's, almost brought an ST4 few years ago, but I don't do that much long distance riding, so it didn't make sense.
So yea, its all up to what bike you find and the price point in my view. Either bike will fit the bill in my view.