Which contains a sentence error?

Cold watermelon on a hot summer day is the best thing, a glass of iced tea is refreshing too.

Cold watermelon on a hot summer day is the best thing; a glass of iced tea is refreshing too.

Cold watermelon on a hot summer day is the best thing. A glass of iced tea is refreshing too.

Cold watermelon on a hot summer day is the best thing, but a glass of iced tea is refreshing too.