I would go 4 oz as well. I did 8oz in an imperial stout and it was nice strong flavor. Lighter stout like this I'd go 4oz.
Those aren't hard rules that you have to follow. They're general guidelines that show you what those yeasts are typically used for. The description for WY1450 is as follows: This terrific all-round yeast can be used for almost any beer style, and is a mainstay of one of our local homebrewers, Mr. Denny Conn. It is unique in that it produces a big mouthfeel and accentuates the malt, caramel, or fruit character of a beer without being sweet or under-attenuated.
To me, that sounds pretty good for a sweet stout. You could leave the lactose in and still go with WY1450. That's actually how I plan to do my next milk stout.