Good, legitimate question! I think quite a few of us work and/or go to school full-time while owning Yorkies. The breeder is right, however, that you can't expect a puppy (
ANY breed of puppy, IMO) to hold it for more than 4 hours or so. Puppies require TONS of attention to begin with when it comes to all kinds of training and attention. It's really like having a newborn child. Expect to be waking up at LEAST once in the middle of the night for the first month or more. It's difficult having to wake up and go to work in the morning but people do it.
Yorkies are not the only breed that will require this kind of attention, specifically in the beginning. Whether you get a Pit Bull or a Chihuahua... they are all pups at one point, who will often be needy, demanding and require a ton of attention and work.
It's actually kinda funny because growing up with all sorts of different dogs, we never had any questions about leaving them for 8 hours or sometimes more. I do find that with a Yorkie, it's different. I can't get myself to leave Jackson for more than 6 hours at a time, but it's typically around 4 hours. I'm lucky enough to have other people living with me that can let him out to potty, etc when I can't be there for some reason. At night, he sleeps for 8-10 hours though. They're very people attached and like to follow you from room to room. Yorkies are smart, and a lot are energetic. Jackson requires a daily walk, trips to the dog park weekly, a game of fetch and/or trick training every night. They're definitely needy dogs.
All dogs are pack oriented and most like to be with their family. With that said.... A LOT of folks work full-time and somehow make it work. Whether it be doggie daycare, a dogsitter pops in 1x during the day, coming home at lunch, etc. I dog-sit for a pair of adult Boxers and even the longest they leave them is usually 6 hours.
Basically -- puppies are a lot of work and require constant attention no matter what breed. An adult dog wouldn't usually require so much.