How accurate is the GPS chip in the iPhone 4?
No WASS or any differential signaling is built into the iPhone. Apple doesn't publish performance data for GPS or the chipsets/algorithms in use.

It's just standard GPS precision (which tries to be precise to 1m on the earth's surface but never is for a variety of reasons) In practice, iOS is usually accurate within 5 meters when the dot shows no circle on the map app (called good or low Dilution of Precision). If Selective Availability (SA) is ever turned on again, all bets are off as the government can intentionally introduce errors of 70 or more meters if desired.

The assisted part of the iPhone GPS makes it faster to get a fix based on assumptions that can be made by trilateration based on cached cell tower and wifi hotspot locations. This doesn't make GPS any more precise or accurate since it's the best signal for iOS location data when it's present.