Seaplane Pilot,
I just picked up a 5050 BIII in fine shape. It's a beast!
And yes, it can play back pre-recorded 4-track tapes as well as older commercial 2-tracks.
Otari still sells the BIII ($5,300 new) and has parts for older models as well. From what I understand, it's one of the most serviceable brands along with Tascam and Studer/Revox.
Legend has it Nirvana's first album (Bleach) was recorded on the 8-track version of the 5050.