Bach is surely a must: two-part and three-part inventions are equal for both hands and fit the level-4 bill.
A lot of baroque music can be useful: Telemann Fantasies feature agility passages for the LH (but never virtuoso textures) being cast very often in two-part texture. The first two are particularly apt:
Cramer Etudes are not easy but very short. In IMSLP you can find the Bulow compilation of 60 Etudes, a good deal of which feature material for the LH. And they are beautiful pieces