Fourth supplement to the catalogue of observed periods of Ap stars

New data on the periods of Ap stars with references are presented (Table 1). 21 further stars are introduced for which a periodic variability has recently been discovered or for which it was known before but it was not reported in previous issues of this catalogue. For many stars also present in previous issues of the catalogue new determinations of the periods are given. Recently attributed variable star names are also quoted. Table 1, notes to this Table and related references are only available in LaTEX form at the CDS via anonymous ftp 130.79.128.5 (filename: DS1178.tex).