You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: either4.go
    path of file: ./repos/mo
    
    the code of the file until where you have to start completion: package mo

import "fmt"

const (
	either4ArgId1 = iota
	either4ArgId2
	either4ArgId3
	either4ArgId4
)

var (
	either4InvalidArgumentId = fmt.Errorf(
    
func (e Either4[T1, T2, T3, T4]) MapArg1(mapper func(T1) Either4[T1, T2, T3, T4]) Either4[T1, T2, T3, T4] {
	if e.IsArg1() {
		return mapper(e.arg1)
	}

	return e
}