How to append to a list in Terraform?

You can use the concat function for this. Expanding upon the example in your question:

variable "foo" {
  type = "list"
  default = [ 1,2,3 ]
}

# assume a value of 4 of type number is the additional value to be appended
resource "bar_type" "bar" {
  bar_field = "${concat(var.foo, [4])}"
}

which appends to the value assigned to bar_field while ensuring var.foo remains unchanged.