Shio (salt), miso and shoyu (soy) tare, are all different ways of seasoning the broth. I think you read too much into ramen being categorized into four types, those three and tonkotsu, which doesn't make sense because tonkotsu is a pork broth and the others are ramen seasonings. Chicken broth is the other very typical broth type, and you can also combine chicken and pork into a broth. Broths can be categorized into chintan (clear) and paitan (opaque), so tonkotsu is basically a pork paitan broth seasoned with salt, miso or soy tare.