This paper introduces ‘fourth places’ as an additional category of informal social settings alongside ‘third places’. Through extensive empirical fieldwork on where and how social interaction among strangers occurs in the public and semi-public spaces of a contemporary masterplanned neighbourhood, this paper reveals that ‘fourth places’ are closely related to ‘third places’ in terms of social and behavioural characteristics, involving a radical departure from the routines of home and work, inclusivity and social comfort. However, the activities, users, locations and spatial conditions that support them are very different. They are characterized by ‘in-betweenness’ in terms of spaces, activities, time and management, as well as a great sense of publicness. This paper will demonstrate that the latter conditions are effective in breaking the ‘placelessness’ and ‘fortress’ designs of newly designed urban public spaces and that, by doing so, they make ‘fourth places’ sociologically more open in order to bring strangers together. The recognition of these findings problematizes well-established urban design theories and redefines several spatial concepts for designing public space. Ultimately, the findings also bring optimism to urban design practice, offering new insights into how to design more lively and inclusive public spaces.