Here are a few websites to start you off with info on NZ. New Zealand Tourism Online http://www.tourism.net.nz/ gives you a lot of material on accommodation, transport and tours. The only frustrating thing about this site is that within set categories (e.g. B&B's, Budget and Backpackers' etc in the accommodation category) the listings are in alphabetical order by the name of the hotel or tour company etc rather than by location. So you will need a decent map to find where some of them are. But you may be able to search by location too on this site- I'm not sure - if not try Google and then cross check if you want to.

You asked about short hiking tours. Hiking ("tramping" in NZ) is very popular and there are literally hundreds of walks. Some of the most famous are the Milford and Routeburn Tracks - both in the South Island, but there are plenty in the N Island too. Many of the tracks have huts on the way where you can stay overnight and if you had your own transport you wouldn't necessarily need to go on a tour.

These two sites will give you a good overview of walking opportunities.

http://tinyurl.com/lkvbl http://www.tramper.co.nz/

You mentioned car hire and you'll find the usual international car rental companies operate in NZ - Avis, Hertz, Budget etc.

In 4 weeks you'll have time to get round both islands and you could consider the third island (Stewart Island - a short ferry trip south of the S Island) too - it doesn't have any roads.

Hope this helps - NZ is a very interesting country with a lot of variety if you like the great outdoors and February (equivalent to August in N Hemisphere) is a good time to go as you get long and (sometimes) sunny days but the Australian and NZ school holidays finish at the end of January so there is less pressure on accommodation etc.

Happy planning, Michael Sydney, Australia