Four Keys and a Cabin by Debby Arthur Warner

"Rachael," Dottie stammered, "I don't think this is some drunk playing a sick game. The psychic was right: We're in danger! This creep's trying to kill us! After inheriting Uncle Billy's cabin in Tennessee, his niece, Rachael, and her friends start out on a fun vacation together, oblivious to the danger awaiting them. Things begin to unravel when Rachael learns her uncle may not have died from drowning, as she was was told. The police believe he may have been murdered! Uncle Billy left four keys with instructions as to what each opened. Not wanting to accept that his death was anything but an accident, she doesn't realize the keys unlock evidence identifying her uncle's killer until it's nearly too late...that someone is out there who knows the secrets hidden in the cabin and will stop at nothing to get them.