Iphone 4 sim into iphone 5s?

I currently have an unlocked iPhone 4 and I use a straight talk micro sim card in it. I want to buy the 5s and unlock it just like how I did with the one I have now. But will I still be able to use the same sim card? Or will it not work?

No, the sim is the wrong size. You will have to get a nano sim.

It depends on which unlock method you used. If you used a SIM card adapter, you would have to buy a new, compatible, one. Go for a factory unlock as you could then update the system to any iOS you like.