4" service guns will work fine for carry if that is what you are wondering. It depends on your body type and the clothes you wear, but I carry an XD40 service all day, everyday. The 4" barrel is not an issue as much as the grip length. My gun prints sometimes with a t-shirt, but not too bad. Getting a holster with adjustable cant like a King Tuk is my next purchase.
As for caliber, get whatever you want. Either caliber is good, I chose the .40 because it has a little more stopping power, but the 9 has more magazine capacity, so its a trade-off.