When a backup spans multiple tapes, the first few data blocks on the tape indicate what sequence number the current tape is (tape #2, #3, etc.) of the overall backup job.
During a catalog operation, it wants to start with tape #1 and go through the whole series until it reaches the end of the sequence. So in your case, "Tape 4" is the first tape used in the backup.
I don't believe the MTF (Microsoft Tape Format) actually annotates on tape the previous tape labels (labels like 'Tape4') used in the backup; which makes me think that you don't need to catalog this tape. To verify this, can you right-mouse click on the tapeand choose "Restore". Does it come back with an error stating the media has not been cataloged, or does it display a restore file selection?
The tapes highlighted in blue indicate media that is overwriteable.