Hot Topics in Contemporary Crystallography by Croatian Association of Crystallographers Humanity is these days just a bit more than a century away from the first structural studies based on the diffraction of X-rays on crystals. Over this relatively short period of time, the level of complexity that the crystallographers are capable of examining has gradually multiplied literally by millions. In particular, over the last decade or so, we are witnessing a remarkable advance in experimental methods and theoretical approaches in crystallography which are putting this scientific branch of ours to the front lines of not only structural chemistry but of natural sciences in general. In order to review and tackle these fascinating recent achievements and developments, we in Croatian Association of Crystallography, have, back in the 2014, developed the concept and launched Hot Topics in Contemporary Crystallography – a high level workshop intended to the utterly ambitious PhD students and postdocs, as well as to the young crystallographers in the early stages of their autonomous carriers, conducting their research within the broad spectrum of disciplines and sub-disciplines related to crystallography. The first edition – HTCC1, was held from May 10th to 15th, 2014 in Šibenik, Croatia and it was a central event to mark the UN proclaimed the International year of crystallography in Croatia. The workshop was a great success – we had 27 students and 6 lecturers from 15 countries. The second edition – HTCC2, was held in Poreč, one of the jewels of the Croatian Adriatic, from Apr. 22nd to 26th, 2017. The workshop gathered 20 students and nine lecturers. The third edition – HTCC3, was held in Bol on the island of Brač (Croatia), from Sep. 23rd to 27th, 2018. The workshop was extremely well attended, as we had 31 students out of which 15 grantees to whom we distributed bursaries in a total amount of EUR 2.560,00 EUR. The fourth edition – HTCC4 was held in the picturesque fishermen village of Mlini, at less than 10 km from the Dubrovnik old town from Oct. 1st to 6th, 2019. The workshop was extremely well attended, as we assembled 58 participants from 17 countries and 3 continents.