Are you sure the aperture size is reduced? :nono: Please go check.
Yes, a 17-40 F/4 lens is always a 17-40 F/4 lens regardless of sensor size.
I think from what grantyale have posted in the past, I can guess that he is speaking from a different angle.
1) If both the full frame user and the Canon 40D user are both using 17mm on this lens, then the 40D user will have to stand 1.6x further away to get the same composition.
As we all know, light fall off by a factor of 1/2 (1 stop exposure) for each squareroot(2) increase in distance.
So the 1.6x further distance for the 40D user means that he is getting the [1.6/squareroot(2)] stops less light than the full frame user who is standing nearer to the subject.
So in terms of exposure, the 40D user's exposure at 17mm @F/4 is the same as the full frame user's exposure shooting the same composition at 17mm @F/6.4. [ i.e. 1.6/(SqRoot(2) x SqRoot(2) x F/4).
The perspectives however are different between the 2 users since the full frame user is nearer to the main subject when compared to the 40D user while the other subjects's relative distance to the main subject do not change with the shooting distance. For e.g. 1 secondary subject 2m behind the main subject will always be 2m behind regardless of your shooting distance. The nearer you're to the main subject, the bigger the difference in its apparent size with the other subjects within the frame.
2) If now, the Full frame user is using a 27.2-64mm F/4 lens instead while the D40 user still uses the 17-40mm F/4 lens, both of them will get the same field of view and therefore have the same composition when they stand together at the same distance away from the subject.
However, the Depth of Field will be different. The full frame will have a much shallower DOF.
The DOF of 17mm @F/4 on the D40 is about the same as the 27.2mm @F/6.4 on the full frame.
So from a DOF point of view, the 17-40 F/4 on the D40 is like a 27.2-64mm F/6.4 on a full frame.
Perspective wise, both users are the same because perspective is only affected by shooting distance and both users now stand at the same distance away.
So the picture taken by 17-40mm @F/4 on the 40D will be the same as the one taken by a 27.2-64mm F/6.4 lens on a full frame in terms of DOF and perspective.
Although the physical aperture (diaphragm) of the 27.2mm @F/4 is bigger than the 17mm @F/4, the exposure is the same for both cameras at F/4 because more light through the bigger lens is spread over a corresponding bigger sensor on the full frame. So in terms of exposure, the F/4 on the 40D from the same distance is the same as F/4 on the full frame and will have an 1.6/sqRoot(2) stops advantage over the F/6.4 on the full frame.
===
So although the 17-40mm F/4 lens remains a 17-40mm F/4, some photographic effects of this lens on a small sensor is equivalent to a 27.2-64mm F/6.4 lens on a full frame.
So I guess grantyale was speaking from a photographic effect angle (point 2 above) rather than a physical dimension or exposure point of view.