Where did you read that the A4 has HD video decoding and the Hummingbird does not.
Samsung claims full HD hardware decoding and encoding on the Hummingbird. I personally suspect the two chips are nearly identical excluding the GPU.
The A4 has a dedicated hardware video decoder in the form of a PowerVR VXD375. The Hummingbird can do it in hardware too, but with its more generalized GPU and/or SIMD units. Both can pump out the video, but the VXD375 lets Apple do it using very little battery. The iPhone 4 gets IIRC around 50% more video play time compared to a Galaxy, even though iPhone 4 has a smaller battery and more pixels to push (higher-res screen).