Is there a way to transfer messages from iPhone 4s and iPhone 5 to iPhone 8?

Shinehah wrote:

Is there a way to transfer messages from iPhone 4s and iPhone 5 to iPhone 8 and still keep the messages on iPhone 8 that are not on the older iPhones? I think restoring an old backup erases the messages that are currently on the iPhone.

That is correct. What you want to do will not work.