Find the 4-digit Numbers! A Fun Number Puzzle from AOPS

Credit to AOPS for the problem in this video (AOPS Volume 1: The Beginning).
Hello viewers! In this video, I explore a fun number puzzle & crack it using simple algebraic manipulation & basic knowledge of algebraic identities. If you enjoyed this video or learned something new, don't forget to like, share your thoughts in the comments section, and subscribe! I'll see you in the next video.
Link to the AOPS book if you're interested (this video is NOT SPONSORED by AOPS): www.amazon.com...
If you have any suggestions for future videos, please do put them in the comments section! Your input matters.