  This volume contains the proceedings of MARS 2020, the fourth workshop on
Models for Formal Analysis of Real Systems held as part of ETAPS 2020, the
European Joint Conferences on Theory and Practice of Software.
  The MARS workshop brings together researchers from different communities who
are developing formal models of real systems in areas where complex models
occur, such as networks, cyber-physical systems, hardware/software codesign,
biology, etc.
  The MARS workshops stem from two observations:
  (1) Large case studies are essential to show that specification formalisms
and modelling techniques are applicable to real systems, whereas many research
papers only consider toy examples or tiny case studies.
  (2) Developing an accurate model of a real system takes a large amount of
time, often months or years. In most scientific papers, however, salient
details of the model need to be skipped due to lack of space, and to leave room
for formal verification methodologies and results.
  The MARS workshop remedies these issues, emphasising modelling over
verification, so as to retain lessons learnt from formal modelling, which are
not usually discussed elsewhere.
