The Molonglo Southern 4 Jy Sample (MS4). I. Definition

We have defined a complete sample of 228 southern radio sources at 408 MHzwith integrated flux densities S_408 > 4.0 Jy, Galactic latitude |b| > 10 degand declination -85 deg < delta < -30 deg. The main finding survey used was theMolonglo Reference Catalogue. We describe in detail how the Molonglo Southern 4Jy sample (MS4) was assembled and its completeness assessed. Sources in thesample were imaged at 843 MHz with the Molonglo Observatory Synthesis Telescopeto obtain positions accurate to about 1 arcsec, as well as flux densities andangular sizes; follow-up radio and optical observations are presented in PaperII. Radio spectra for the MS4 have been compiled from the literature and usedto estimate flux densities at 178 MHz. The strong-source subset of MS4, withS_178 > 10.9 Jy (SMS4), provides a southern sample closely equivalent to thewell-studied northern 3CRR sample. Comparison of SMS4 with 3CRR shows areassuring similarity in source density and median flux density between the twosamples.