It's hard when everyone has an opinion...here's mine! lol
I feed about the same amount twice a day. When my boy was a pup he was fed 3 times like you are doing. His food times vary by hours sometimes, but it's around the same time every day. I am not going to stress about a fixed time...life happens & dogs will survive by eating early or later on some days
I personally don't like to free feed. The term can mean different things to different dog owners. To me it means not measuring food and allowing a dog to eat whenever.
4 cups a day sounds about right. We want our GSD to grow slow & steady. You should be able to feel ribs, they should not be protruding. My boy went through a weird teenage awkward stage around 7 months...nothing looked proportioned
To this day I hear that my boy is small for a GSD....but I know better. He's a good weight & height....also not over weight which unfortunately joe public is not use to seeing.