The first opportunity to cite a source with more than 3 authors requires you to mention all the authors and then, subsequent to that, you may use only the first name followed by et al.
In-text: Shapiro, Bethu, Ndlovu and Wilcox (2000) provided an analysis of ….. or at the end of the sentence ... A new media planning format has been introduced…. (Shapiro, Bethu, Ndlovu and Wilcox , 2000).
In-text subsequent to the first citation: Shapiro et al. (2000) provided an analysis of ….. or at the end of the sentence ... A new media planning format has been introduced…. (Shapiro et al., 2000).
Reference list: Shapiro, D., Bethu, R., Ndlovu, B. and Wilcox, S.R. (2000) “A new media planning format”, Journal of Media planning, vol. 10, no. 5, pp. 454-65.