Water Filtration System
Antibiotics
Container of Fuel
Rope
The answers:
1. Water Filtration System - again, you can only survive for 3 days without water, and you may not have access to a safe source. This will allow you to make any water safe for consumption.
2. Rope - this can be used for a myriad of things, from setting traps to catch food or to get yourself in and out of a safe place where zombies can't reach you.
3. Antibiotics - although these won't be helpful against the zombie virus, there are a myriad of other infections that could take you out. These can come from bacteria that get into your system from something simple as a scratch. There's no way to fight an internal war and an external one, so keep yourself healthy.
4. Container of Fuel - this could be helpful if you have a vehicle that requires fuel or for starting fires, but only if you have a vehicle or something to light it with. With all the other items that could potentially be more helpful, this one is better left behind.