Type WS is preferred for its superior lubricity and durability, but T-4 can work albeit with a shorter life and crisper shifts, providing it is changed frequently.
Again, if WS is available, it should be used, no doubt.
The majority of the fluid is in the torque converter and clutch packs, which are not drained when the pan is drained.
Typical drain and refill on a Toyota auto trans is only a few quarts (liters) around 2 to 5, with the total capacities ranging from 7 to about 15 quarts (liters).
The fluid level requires a certain temperature to be checked, but otherwise it a fairly straightforward procedure like the manual indicates.
Some people use a machine hooked through the trans oil cooler pipes to change out all the fluid, while others have hooked up simple home made pipes and done the same if you are so inclined.