Hi ya Jeff,
The way I understand it is that the feeling of fullness comes with time. At 4 weeks out, stomach nerves cut during surgery are still healing and it can take 2 - 3 months for the pouch to brain signal to kick into gear to give you that full feeling. Also, I suspect that @ 4 weeks you are still eating soft foods maybe even purees and these tend to slide through the pouch fairly easily. So anything soupy will empty out quickly giving you greater capacity to eat. I figure this 'never feeling full' business will change as you move to a more solid diet.
My surgeon explains that today's bypasses use the hard to stretch part of the stomach so I don't think you need to be worrying about stretching the pouch (though pouch abuse can lead to stretching of both pouch and stoma - stoma stretching being more of a worry) The best thing to do while you wait for the 'full' sensation to kick in is to perhaps measure your food according to guidelines you were given, if any.