What are the chances of a 4mm kidney stone becoming obstructive?

Likely: Kidney stones will grow is left untreated. A 4mm stone can obstruct the ureter and cause severe pain. A stone of this size can likely removed endoscopically. See this site for more info. http://www.mayoclinic.com/health/kidney-stones/ds00282.

Ureteral stone: The size of the stone does not correlate well to the intensity of the pain. The nearly unbearable pain of "ureteral colic" is from spasm of the muscular ureter. Although 50% probability to pass on its own within a few days, a 6mm stone sometimes will get stuck and fail to progress. Usually after 5 days, i recommend having the stone extracted with a ureteroscope as an outpatient.