Four Massachusetts community hospitals are investigating how thousands of patient health records, some containing Social Security numbers and sensitive medical diagnoses, ended up in a pile at a public dump. The unshredded records includedpathology reports with patients’ names, addresses, and results of breast, bone, and skin cancer tests, as well as the results of lab work following miscarriages. By law, medical records and documents containing personal identifying information must be disposed of in a way that protects privacy, and leaving them at a dump is probably illegal, privacy lawyers and hospital officials said. Violators face steep fines. A Globe photographer discovered the records July 26 when he was dumping his trash at the Georgetown Transfer Station. When he got out of his car, he said, he saw a huge pile of paper about 20 feet wide by 20 feet long. Upset that the paper wasn’t being recycled, he looked more closely.