Update lecture_4.md - added LG to lecture 4B

src/lecture_4.md

@@ -260,6 +260,7 @@ Also a sanity check: when the energy is close to the bottom of the band, $E = E_
     - formulate equations of motion for electrons or phonons in 1D for systems with more than one degree of freedom per unit cell.
     - solve these equations to arrive at the dispersion relation.
     - derive the group velocity, effective mass, and density of states.
+    - explain what happens with the band structure when the periodicity of the lattice is increased or reduced.
     
 ### More hoppings in LCAO