Four-card-section using responsive grids

I know my solution doesn't match exactly with the sample provided design. I have been stuck with the layout of four cards throughout the day. I am still not able to get the right layout. Please feel free to suggest me where am I doing wrong and how can I correct my CSS code.

I would be grateful if you will have look at my code and pin-point the exact point which is preventing me from achieving the design layout. Eagerly waiting to receive some great insights!