writing LTO-3 tapes with a LTO-4 drive

I have restored LTO3 tapes that were written to on an LTO4 device. I used an LTO3 device to do the restore and actually had no problems.

The tape was only ever used in the LTO4 drive tho and hadnt been appended to from an LTO3 device.

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.