The youngest stellar populations of a 'J'-shaped region inside the supergiant shell (SGS) LMC4 have been analysed with CCD photometry in B, V passbands. This region consists of 2 coherent strips, one from the east to the west reaching about 400pc across the OB superassociation LH 77 and another extending about 850pc from south to north. The standard photometric methods yield for each of the 25 CCD fields a colour-magnitude diagram (CMD) which is used for age determination of the youngest star population by isochrone fitting. The resultant ages lie in the range from 9Myr to 16Myr without correlation with the distance to the LMC 4 centre. We therefore conclude that there must have been one triggering event for star formation inside this large LMC SGS with a diameter of 1.4kpc.