Slicer 3 achieves a more pleasing result with the same workflow. The transfer function makes more sense.

Fixed in r19343:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=19343

thank you, thank you

Unfortunately, the specifics are not quite right. I used the sample data set CTA-cardio on mac nightly march 4:
min -322 max 689

"What I get" shows that the ramp is not positioned properly. "What I should get" shows a ramp which I adjusted manually to the correct beginning and end.

Slicer3:
Go to Volumes module and Select CT Abdomen preset
WL: -135Min 215Max (350W 40L)
Threshold: -959Min 3532Max
Go to Volume Rendering module:
Color map: -135Min 215Max
Opacity map: -135Min 215Max -> not consistent with Threshold
Synchronize with Volumes:
Color map: -135Min 215Max
Opacity map: -959Min -954Max -> inconsistent with first values when opening volume rendering, it is very confusing.

Slicer4:
Go to Volumes module and Select CT Abdomen preset
WL: -135Min 215Max (350W 40L)
Threshold: -1024Min 3532Max -> it should be -959Min, I'll fix it
Go to Volume Rendering module:
Color map: -135Min 215Max
Opacity map: -1024Min 3532Max -> consistent with Threshold
Synchronize with Volumes:
Color map: -135Min 215Max
Opacity map: -1024Min 3532Max -> still consistent

The problem with the initial values in Volume Rendering in Slicer3 are not consistent.
In addition, once the transfer functions are set, there is no way to go back to those values, it's a one time shot.
It really is confusing to me.

Alternative suggestions (not orthogonal):
Presets in Volumes module can set Threshold values. CT-Abdomen sets WL and Threhsold to -135Min, 215Max (effort index: 1)
Add Threshold mode in Volumes module to "follow Window/Level" and make it the default (effort index: 2)

It worked properly in Slicer 3. I just want back what I already had in Slicer 3, No more, no less.

Difference I have found between Slicer 3 and Slicer 4:

In Slicer 3, Volumes module display settings will not be used by volume rendering unless "Sync with volumes" button clicked. Whereas, in Slicer 4, volumes module display settings will be used as default for volume rendering. What Ron seen in Slicer 3 is volume rendering done with its own opacity ramp (SetupVolumePropertyFromImageData()), whereas in Slicer 4 the opacity ramp is generated based on volumes module (UpdateDisplayNodeFromVolumeNode() called in createVolumeRenderingDisplayNode).

Another issue I found is that sometimes VolumeRendering shows complete dark renderings. But any update to 3D view will revert it back to normal rendering. That's probably why Ron is seeing dark images

I understand, and I will do it.
Now, going further, how do you display a 2D slice in 3D view with the air transparent ?
You first show the 2D slice in 3D, go to Volumes, select CT-Abdomen preset, and you need to play with the threshold slider all the way to ~ -135.
Question: wouldn't it be nice (meaningful) if it was already at -135 for you ?
The min of the window/level is likely to be the min of the threshold.
If you believe though, then it makes the initial look in Volume Rendering the same as in Slicer3 and it is the same look when "Follow Volume" is ON.

I like the idea of "The min of the window/level is likely to be the min of the threshold.". Would it be possible to added a "Follow W/L" item in the combo box showing "Off", "Auto", and "Manual" so that user can apply W/L setting to thresholding?

Fixed in r19573:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=19573

The rendering now looks good. The scalar opacity map: the chart does not look correct. See most recent image.