Holy cow, I wrote a book!
On Windows NT-based operating systems,
process and thread IDs happen always to be a multiple of four.
Is this just a coincidence?
Yes, it's just a coincidence, and you shouldn't rely on it
since it is not part of the programming contract.
Windows 95 process and thread IDs were not always
multiples of four.
the reason that
kernel handles are always a multiple of four
is part of the specification and will be guaranteed
for the foreseeable future.)
Process and thread IDs are multiples of four as a side-effect of
The same code the allocates kernel handles is also used to allocate
process and thread IDs.
Since kernel handles are a multiple of four,
so too are process and thread IDs.
This is an implementation detail,
so don't write code that relies on it.
I'm just telling you to satify your curiosity.