We generalize a phase transition between three-dimensional hot flat space and a cer- tain type of flat space cosmology to four dimensions. To do so, an analogue of this cosmology is constructed in four dimensions and novel flat space boundary conditions are established, that differ from the usual boundary conditions of asymptotically flat space in four dimensions. Also we construct the Lie algebra of asymptotic Killing vectors that preserve these boundary conditions. A generalization of the phase transition can then be found straightforwardly. We will find that there are some differences in possible interpretations as compared to the three-dimensional version, which will also be discussed.