The proper way to sketch a half wall is to use either the room tool OR the wall tool (i.e. wall protruding into a room). Once the wall is drawn, click on that wall.
Go into the wall properties screen.
Under "Missing end to end" select YES
The system will automatically change wall thickness to 0". Set it back to 4"
Then at "Opening Base Height", set the wall height (4' or whichever height it is)
The illustration above was put in using the wall tool. Try it, go into 3D, and it will be precise. This is not exactly intuitive, but is simple once you try it, and your variables will all be exactly correct.
Chris Hatcher, AIC, Xactimate Certified Trainer