Assume that there is a dielectric material with a mass density of ρ0 observed in the dielectric-rest frame. And further, it is assumed that observed in the lab frame, v(x,y,z,t) is the velocity distribution, β=v/c is the normalized velocity, and γ=(1-β2)-1/2 is the relativistic factor, with c the vacuum light speed. In my opinion, ρ0 is a Lorentz scalar, and γ(v,c) is a Lorentz 4-velocity, and thus ρ0γ(v,c) also is a 4-vector. My question is: Can ρ0γ(v,c) be defined as the momentum density 4-vector of the dielectric material? What I mean is: If ρ0γ(v,c) is defined as the momentum density 4-vector, is it compatible with the principle of relativity? Note: I do not claim ρ0 is a constant, otherwise the material would be rigid, not consistent with the relativity.