How can I estimate the spatial resolution of consumer grade electronics?
Consider the following figure:



The cameras are separated by baseline 2b - say 10cm, so b=5cm.  Your focal length of 4" sounds looong (about 100mm) for a webcam, but hey, let's go with it.  f=100mm (or also 10cm!). 

From $xr-xl = \frac {2bf}{z}$ an object at 10m away will have a pixel difference between the two cameras ($xr-xl$) of 0.1*0.1/10 = 0.001m.  So, if your pixels are 5 micron square $xr-xl$ is about 200 pixels. 

Which is a lot - large disparities are expensive to search for.  

With a more normal focal length of say 5mm, your disparity is more like 10 pixels.  11m is about 9 pixels, so your distance resolution is about 1m @ 10m.

If you have a specific camera in mind you can run the numbers for your particular geometry.