The iPad 4's CPU is getting pretty long in the tooth now. Fortunately, with the iPads (and tablets in general) the hardware has far outpaced the software so there aren't a lot of apps, besides a few high end games, that take advantage of the fastest hardware. As long as you aren't looking to be a heavy gamer, the iPad 4 still performs quite well in 2015.
iOS 9 has little difficulty running on older hardware. My experience on my 5th gen iPod Touch has generally been more positive on iOS 9 than it was on iOS 8.