You are presumably going to make the tanks as thin-walled (more generally low-mass) as you can. To understand this you need to know what stresses the tank walls are there to resist. To a first approximation, there are two: the pressure of the fuel inside (if the fuel is a gas to a liquid with significant vapour pressure) and the forces due to the acceleration of the rocket. If you are going to alpha centauri you are almost certainly limited by the maount of propellant you have, rather than the acceleration, so we can assume you accelerate very gently and ignore those stresses.
The forces due to fuel pressure can be modelled easily enough. We can assume the tank is spherical (the maximum volume for a given surface area, and a strong shape for resisting pressure). Suppose the radius is $R$ and the pressure $P$ and the tank thickness $t$. The tension in the skin of the tank is fairly easily calculated to be $PR/t$. For a given tank material this is limited to some yield strength $Y$ so we get $t = PR/Y$.
So now the mass of the tank is $4\pi t R^2\rho$ ($\rho$ is the density of the material) which is $4\pi PR^3 \rho/Y$.
For gaseous fuel, the mass of fuel is proportional to the volume times the pressure, so to $PR^3$ so the mass of the tank is actually a fixed proportion of the mass of the fuel, depending only on properties of the tank material. Given that, we may as well use small tanks and drop them as we go along.
For liquid fuel, $P$ is a constant, depending on temperature and fuel but teh above calculation still works, although if $P$ is small, the tension in the tanks will be small and it will be hard to keep other forces low enough.
For solid fuel (well below its sublimation point) no tanks at all are needed.