Add HW4 to homework table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: alanedelman

README.md

@@ -98,6 +98,7 @@ Final Project reports due: May 11
 |1| [PINNs](https://github.com/mitmath/18337/blob/master/18.337%20%202026%20homework%201.pdf)| 2/11/2026 |
 |2| [Performance etc](https://github.com/mitmath/18337/blob/master/18.337%202026%20homework%202.md)|  Thurs 3/5/2026 ||
 |3| [Project Proposal & Adjoint Eqn ](https://github.com/mitmath/18337/blob/master/18.337%202026%20hw3.pdf) | Friday 3/20/2026 ||
+|4| [HW4](https://github.com/mitmath/18337/blob/master/18.337%202026%20hw4.md) | ||