I know that in the US, people prefer to use the simple past over the present perfect.
Context: I would like to express my opinion about a country and give a recommendation to my friend:
(1) I have visited Thailand . It is a nice country. You should visit it.
(2) I have visited Thailand twice. It is a nice country. You should visit it.
According to my grammar books, I should use present perfect as in (1) and (2).
But when I asked a US speaker, he told me that I also can use the simple past as in (3):
(3) I visited Thailand twice. It is a nice country. You should visit it.
So I wonder, if I can use simple past without the word "twice" as in (4) without time marker or not:
(4) I visited Thailand . It is a nice country. You should visit it.