In Angular2 you could have a folder /data/ and a json file there and you could access it at localhost:4200/data/something.json.
This is no longer possible in Angular
I was facing the same issue, where my Observable Angular service was located at inside 'src/app/' folder and it was trying to load a local JSON file. I tried to put the JSON file inside the same app folder, inside a new 'api' folder, used various kinds of relatives/absolute routes, but it didn't help and I got 404 error. The moment I put it inside 'assets' folder, it worked. I guess there is more to it?
Maybe this link helps:
COMPONENT-RELATIVE PATHS IN ANGULAR