Four things for an apocalypse? Then I'll take:
1) A copper-bottomed pot. For boiling water, drinking water, cooking food, improvised melee use, storage, etc.
2) A lighter. To start fires with.
3) A spiral notebook, with mechanical pencil. For writing down important info, etc.
4) A multitool. For opening cans, preparing firewood, cutting wires ropes and the like, miscellaneous maintenance, as a thing to mess with when bored so as not to waste the lighter or pad, etc.
EDIT: I, of course, assume that clothing need not be specified.
1) A copper-bottomed pot. For boiling water, drinking water, cooking food, improvised melee use, storage, etc.
2) A lighter. To start fires with.
3) A spiral notebook, with mechanical pencil. For writing down important info, etc.
4) A multitool. For opening cans, preparing firewood, cutting wires ropes and the like, miscellaneous maintenance, as a thing to mess with when bored so as not to waste the lighter or pad, etc.
EDIT: I, of course, assume that clothing need not be specified.