What are L4 and L5?

They could be several things. Many different systems use an alphanumerical combination to record and file thinsg, and the terms L4 and L5 can refer to some specific things depending on what field you work in. Examples commonly used are the lumbar verebrae L4 and L5 in anatomy, and the LaGrangian points L4 and L5 in astronomy.

The lumbar vertibrae L4 and L5 are the sciatic region of the back and are the most common area of disk compression failure, leading to chronic sciatica and other lower body nerve compromise.