Differential cross sections, analyzing powers, and spin rotation parameters for elastic scattering of protons on He-4 at 297 MeV have been measured over a wide angular range ( 12 degrees less than or equal to theta (lab) less than or equal to 66 degrees). Experimental data have been microscopically analyzed with relativistic impulse approximation calculations. The spin observables up to medium scattering angles (theta (c.m.) less than or equal to 40 degrees) are predominantly sensitive to the ratio of the volume integral of scalar density to vector density. With a ratio of 0.92, which is significantly smaller than those for heavier nuclei (0.96-0.98), both spin observables as well as the differential cross section have been quantitatively reproduced. According to nuclear matter calculations, the small ratio reflects a high mass density. At backward angles, the experimental cross sections are larger than any of the values calculated with realistic density distributions.