Can i remove my sim card from my galaxy s3 and put it into the iphone 4s?

Will it work?
I have At&t Samsung galaxy s3 4G LTE

Yes you can, both are micro sim.

Yes you can put because both work on Micro sim but now point is about your phone is carrier protected or not first check this!

Yes you can, but your cellular data might not work. You'll need to have your service provider change your plan to an iPhone data plan (same price) instead of the android one.

Both use microSIMs but the more important question is if both are on the same network, well the 4S anyway.

If the 4S is on AT&T or unlocked then yes it will work no problem.