After the successful organisation of the first three workshops (SEEFM'03 - Thessaloniki, SEEFM'05 - Ohrid, SEEFM'07 - Thessaloniki) the 4th South-East European Workshop on Formal Methods aims to continue bringing together researchers of the Formal Methods community, not only from SEE region but from the rest of the world as well. The workshop will take place on the 5th of December, 2009, and is organised by CITY College and SEERC. This year's theme is twofold and involves:"Formal Methods for Web Services" and "Formal Methods for Agent-based Systems". The workshop will include two keynote addresses by Prof Mike Hinchey of Lero, Ireland, and Prof Wolfgang Reisig of Humboldt-Universitat zu Berlin, Germany.