Can you trade an IPhone 4s for a iPhone 6?

Some Apple Stores let you trade in your phone for credit towards a new iPhone. You need to call your local store and find out if they offer this.

Not with Apple. They do not have a trade-in program. They do offer recycling for a credit, but the amount is very small. You are better of trying to sell it yourself on eBay or other method, and then use the proceeds to purchase a new iPhone. Some carriers do trade-ins, you can contact them if you like to see if they do and if you qualify.