Kind of a complication question

Ok so here's the deal. I ordered a 5s this morning when they became available. I had hoped to go get one at the store but realized that I wasn't going to be able to get off work and didn't want to take the chance they'd be sold out by this evening. The problem is I recently sold my iPhone 5 and need to ship it today. Worst case scenario is I simply go without a phone until next week when the 5s arrives but I'm hoping to avoid that. I have an old iPhone 4 lying around and am wondering how big of a pain it would be to get that activated on my line for the next few days. Since it's not an LTE device I know it isn't as easy as swapping the sim card. My question if I do activate the 4 temporarily will I still be able to use my same sim when my 5s gets here next week? What will the procedure be to go from my old 4 to my new 5s once it gets here?