15:20-25 is a safe bet. 4:04 converts to a 15:16.
She has no prior cross country times that remotely suggest she could run that fast, but those are from last year. Even normal high schoolers will see dramatic drops in their times after a year or two of consistent training. And, Mary Cain is not a normal high schooler.
Further, she's beaten some pros who could easily run 15:30s. So, she has to know that by inference that she can run at least that fast if not faster. Also, Salazar wouldn't put her in such a race if he didn't think she was ready. They've probably done some 5k stuff in workouts that suggest she could pop a good one.
Lastly, Mary Cain has some great intangible instincts when it comes to racing. She rises to the level of the race she is in. I see no reason she wouldn't be ready to rise to the occasion here and run a solid 5k that a 4:04 runner is capable of doing.