2004.09.17 00:09 "Re: [Tiff] BigTIFF Tag Value Count issue", by Bob Friesenhahn
Argument pro 4 bytes is that this keeps all 8 byte values in the TIFF file perfectly aligned on multiples of 8.
This may be a poor argument, since alignment inside a file really might not be that important.
My opinion on this is that alignment inside a file is just as important/useful as alignment in memory since the file may be memory mapped, and then all the limitations which apply to memory access also apply to file access via that memory map.
It is not necessary to declare that at the value needs to be 8 bytes in order to accomplish desired alignment since you can simply add in 4 unused pad bytes.