Yeah, a four dot ellipsis is mostly used to indicate omitted text in a quotation after a sentence ends, but can also be used to convey pauses, silences, leading statements & unfinished thoughts that occur after a sentence ends. To use your example:
Alf: "Are you sure Mary isn't the park poo-jogger?"
Bazza: "Well, today, Mary went to the park...."
Alf: "And? Did you see what she was up to? It must be her, right?"
Alf: "Aww, you're such a tease - spill the beans!"
If the context of your example indicated that more information could or should have been forthcoming past the end of the sentence but was omitted, the four dot ellipsis could adequately express that. As to whether the four dot ellipsis should be a period-space-3-dot-ellipsis or four dots in a row seems entirely a stylistic choice.