They are compatible up to a point. LTO4 drives can read and write LTO4 & LTO3 and read only for LTO2. So if the original backup was done with a LTO2 or LTO3 drive it has the ability to read the tapes. If the backup was done with a LTO1 drive then the LTO4 drive cannot read it but a LTO3 or LTO2 drive can. It all depends upon the type of drive used for the backup. If you are not sure on the type of tape check out LTO_tape.
Other possibilities are the media was not stored properly and can no longer be read by any drive, or it was backed up with some other application which the current backup application cannot read.