Canned Food/Can Opener
Long-sleeved Shirt and Pants
The answers are:
1. Canned Food/Can Opener - while you can survive for slightly longer without food than you can water, it is still essential to live. It also gives you energy and the ability to run from zombies.
2. Long-sleeved Shirt and Pants - while being trapped in the zombie-apocalyptic world, you'll need to protect yourself from the elements. Imagine how hard and painful it would be to fight off zombies with a horrible sunburn.
3. Handgun/ammunition - I'm pretty sure this one doesn't need an explanation, but it could be used to kill zombies or hunt for food if your canned food runs out.
4. GPS - while the idea of having a GPS to map your location is enticing, it's impractical. There's no guarantee you'll be able to connect to the satellite or have a power source to run it. It would just be a waste of space in your kit.