In 4 min the HQ calling the support , has to : locate precisely the target position on his map, estimate precisely the azimuth form the mortar to the target, then estimate the range from the mortar to the target, lookup in a range table the corresponding elevation for the mortar. And once the mortar squad receive this information , they must setup their mortars with the right elevation and deflection , take the shell out of their casing, put the right amount of charges on the shell and finally fire the shell. 4 minutes doesn't seem that much for doing all this.