You can get either canned or dried fruits and veggies that last longer un-refrigerated and have some nutrition to them. Obviously there’s power bars as well. Dry cereal if you don’t mind eating it dry, makes a decent snack too, or bring powdered milk if you do want to eat it with milk. I’ve heard freeze dried foods are popular with backpackers. Bananas last long enough if you get them fresh enough. I think there are other fruits and veggies that you don’t need to refrigerate for them to last long enough, like tomatoes, cucumbers, oranges, pears, kiwis, apples, etc, just as long as you eat the whole thing in one go cause they don’t last as long once they’re cut or bit into. Oh and fruit cups are also an option. And of course there’s bread, but if you want some variety there you can make/buy some banana bread to take with you as well. Popcorn also lasts quite a while if kept in a sealed bag. You can also eat ichidan or other dry packaged noodles dry, just break them up into smaller pieces in the bag, take out the flavour packet, and either pour the packet into the bag, or pour the noodles and flavouring into a bowl, and mix it up. Tastes pretty good, and makes for a good snack. Not very nutritious, but cheap and easy. There’s also pop tarts, which can be eaten cold.

Those are the ones I can think of off the top of my head. You don’t need to take all of them for just 4 days, but it’s nice to have options.