I'm still reviewing this, and my terminology is still a bit off, but I would approach this as follows....(Keep in mind I could be mistaken, and I'm sure one of the wonderful CCIE's on here will let us know)
Switch 3 is the Root, therefore all interfaces on that switch are FWD
Switch 1 and Switch 4 both have RP's tied directly to the Root,
This leaves us with the following interfaces/switches
The port that would block is SW2, Gi0/2 if you assume all req's are =, it will come down to port #, and the lower port will fwd.
RSTP Process for port discarding is
1 - Lowest Cost Path to root
2 - Lowest switch ID
3 - Multi-connections to root use lowest priority (if same see 4)
4 - If 1-3 don't solve, then the port used will be the lower port # (So the higher port will be discarding).
Switch 1's cost to the root is 19 via its direct connection, so this is the cost it reports. Switch 2 on the other hand has to report a cost of 23, its Gi interface connecting to switch 4 has a cost of 4, but switch 4's connection to the root has a cost of 19, so the total cost is 23. Even though switch 4 has a Gi interface to the root, the cost of that link is 19, because of the Fa interface on the root. Least cost path to the root wins.