The author visited four countries или The author has visited four countries
The author visited four countries.
Так как действие совершено и закончено в прошлом, и не имеет видимых результатов в настоящем.
"The author has visited four countries" is correct because you are stating a fact. You need to use the perfect tenses when talking about a past event which continues to the time being described.
Sorry I meant The asuthor visited for countries is correct