I don't think block and chunk are the same thing, if you follow the link to the WAVEFORMATEX structure details and then onto the MSDN page you'll find the following;
Block alignment, in bytes. The block alignment is the minimum atomic unit of data for the wFormatTag format type. If wFormatTag is WAVE_FORMAT_PCM, nBlockAlign must equal (nChannels × wBitsPerSample) / 8. For non-PCM formats, this member must be computed according to the manufacturer's specification of the format tag.
So for a 16bit stereo sample you get (2 x 16) / 8 or 4 bytes per sample.
A 'chunk' on the other hand looks to be header + audio data, which will of course be larger than 4 bytes ;)
That's exactly what caused my confusion in the first place! Pulling apart wav-files I've been extracting various chunks (look here), which makes the following
If the IMA data is monaural, an individual chunk of data begins with the following preamble:
bytes 0-1: initial predictor (in little-endian format)
byte 2: initial index
byte 3: unknown, usually 0 and is probably reserved
make no sense to me, at least if they are referring to the header+data-chunks. But I would think they're actually referring to the blocks - on the other hand I heard/read/can't remember somewhere that IMA ADPCM cannot be decompressed per-block which would contradict each block having a header, that could've been in a dream though.. CONFUSED !
Edit: Uh, I'm running so many parallell possible solutions to this little problem - I'll go back to the one I was working on when posting the images above and apply my new knowledge. This thread can be put on ice for now.