If 4+ weeks since breeding you should start seeing changes very soon. She's also far enough along to do an ultrasound to confirm pregnancy and get a rough count of pups, which is what I'd do.
The early stimulation stuff you're inquiring about is the Biosensor/Superdog program of early neurological stimulation. A lot of breeders do it, or at least a version of it, and we always have as well. Does it help? Who really knows. But it certainly doesn't hurt and the scientific principles behind it are sound. Though the key is to expose them to mild stress, but not too much stress, so it's important to not overdo it. Definitely a case of where the program says 3 seconds, 5 or 10 seconds is *not* better.
Carmen Battaglia's article describing this is all over the internet. Here's one website with it:
Early Neurological Stimulation, Tehillah German Shepherd Dogs
Ian Dunbar's rules of 8 (another you can find by googling) has some pretty good guidelines as well. It is important to get them on different footing surfaces, exposed to different sounds and smells and textures and locations. Puppy obstacle courses, mini agility courses, wobble boards, stairs and other challenges like that when they get a bit older are great too. Just be careful not to overdo it or throw anything at them that is beyond their age and ability to handle because experiences should be positive, or at most very slightly stressful and easy for them to work though. Don't want to traumatize them.
We do the litterbox thing. It does definitely help, both with the pups learning to discriminate potty areas, and just with cutting down the mess. Their indoor area is divided. A litterbox area and then another area with blankets on the floor where they eat, sleep and play. We used cedar chips for years, but with our last litter after a couple different breeders recommended it we tried a pelletized horse stall bedding product called Equine Fresh and did like that stuff a lot better so will probably continue to use that instead of the chips in the future.
Some pics of the indoor set up:
We also have a doggy door into an outdoor kennel and before long they start going out there for potty breaks too. The door isn't always open as we close it at night for the first few weeks until we know they can see and navigate well enough to find their way back inside when it's dark, and also keep it closed if the weather is really nasty.
We start crate training before they go to their new homes as well, and that really helps.
We have an article on our website that provides a bit more detailed overview of how we raise our pups, so you might get some ideas from that too:(Wildhaus Kennels Raising Working German Shepherd Puppies)