4 flashes means the hard drive has failed...not at all uncommon for these older units. Once they're powered down after years of spinning then powered up again they frequently stop working. Best replace the unit with one of the flash based ones (the hard drive is not a regular PC one, it has Pana-specific ROMS/firmware and replacements are very expensive).
All of the TVS series are nearly identical in programming - only ones that are different are the newer TVA's which have USB and an improved interface with their own software. The TVS50 actually came out after the 75....main differences are the 50 is flash-based (rather than HDD), has only 32 mailboxes (vs 64), has the newer voice prompts, and lacks the "company greetings" feature (you just have to use different custom service prompts instead) found in the 75/80/90/95/& up.
I'd try to locate a used TVS50, 80, 90 or 95. Those are all flash-based units that are cheap and readily available....much quieter than the HDD-based ones and I've never had one die.