In order to get the 4hr UH from a 2 Hr UH, we have to lag the 2 Hr UH by 2 hours and get the DRH. This DRH will be corresponding to the ER of 2 cm, hence to get the 4 Hr UH we have to divide the ordinates of DRH by 2.
The ordinates of a 2-hour unit hydrograph for a catchment are given as

The ordinate (in m³/s) of a 4-hour unit hydrograph for this catchment at the time of 3 h would be______