Four nights on the south island is really not enough time, but you take what you can get, right? I love both places. Christchurch is definitely more of city and has a bit of an English vibe. Queenstown is the place if you love spectacular scenery, adventurism, and great hiking. I haven't been in a few years and hear it's getting more and more crowded, so you could consider some other smaller towns around Q-town, like Wanaka or Te Anu (great access to the trails there). You really can't go wrong in NZ. For my money it's the most spectacular place on earth. Have a great trip.