There is one exception to only pouring smaller amounts such as 3 or 4 ounces from a bottle.
If a guest orders a separate wine-by-the-glass, and it is your job to go to the bar and pour that wine, you should pour the full amount that your manager or bartender says is the restaurant’s standard pour. Usually, that is either 5 or 6 ounces.
The guest who is paying for a separate wine and not being shared from a bottle expects a full pour since they are being charged separately.