Apparently LTO-4 drives can both read and write LTO-3 tapes. Are these tapes fully compatible with LTO-3 drives? I'm interested in real life experience more than theoretical vendor-touted compatibility :)
Yes. It's not just a vendor touted compatibility, it's a requirement that's tested as part of the LTO Ultrium certification process. All LTO-4 drives must read LTO-2/LTO-3 and write LTO-3. Specifically see questions 27 and 28 of the LTO FAQ.
That said, I've had no problems moving LTO-3 tapes between my Exabyte Magnum 224 (aka Tanberg Data T24) autoloaders. One has an LTO-4 drive, one an LTO-3; everything worked as advertised.