When a provider has been updated outside of Terraform, (for example in Terraformer) Terraform can get out of sync with that provider version.
terraform init works, but
terraform plan throws the error message:
Error: Failed to instantiate provider “aws” to obtain schema: Incompatible API version with plugin. Plugin version: 4, Client versions: 
To update Terraform to be able to use the new provider execute
terraform init -upgrade