I want to use my old iPhone 4S for iMessage but it will not activate.

See if any of the information in this support document help. iOS: Troubleshooting FaceTime and iMessage activation

Okay, if you did everything listed there, then the final thing you did was to contact Apple Care. That is the final step. If they couldn't answer it, then no one here would be able to.

I cannot be sure of that, however for out of warranty devices there is generally a fee. If you have an Apple store close by, you can always call for an appointment at the Genius Bar and go in for help too.

I'm not an expert on this, but it is my understanding that iMessage, like text messaging, sends posts to a specific phone number. If that number is now attached to your new phone, that's where the messages will go.

Re-activate your iPhone as an iPod touch

Using an iPhone without a wireless service plan

Since it became an iPod touch, you do not have phone number for iMessage.

You use an e-mail address.

You can use iMessage & FaceTime.

iPhone 4S is a worlds iPhone, it has a micro-sim slot (can you not see on the right side of iPhone?).

The sim need not be activated but must be in.

iPhone 4S uses a micro-sim, iPhone 5/5c/5s/6/6+ uses nano-sim.

You can use an old sim or borrow from someone.