  This paper is a write-up of the ideas that were presented, developed and
discussed at the fourth International Workshop on QCD Challenges from pp to AA,
which took place in February 2023 in Padua, Italy. The goal of the workshop was
to focus on some of the open questions in the field of high-energy heavy-ion
physics and to stimulate the formulation of concrete suggestions for making
progresses on both the experimental and theoretical sides. The paper gives a
brief introduction to each topic and then summarizes the primary results.
