The lanthanide contraction is responsible for the fact that:

Group 4 contains both $\mathrm{Zr}$ and $\mathrm{Hf}$ (d-block). $\mathrm{Zr}$ atomic radius (160 pm) is relatively close to $\mathrm{Hf}$ (159 pm).
The lanthanide contraction causes $\mathrm{Zr}$ and $\mathrm{Hf}$ to have comparable characteristics. With an increase in atomic number and effective nuclear charge, the size of $\mathrm{Hf}$ and $\mathrm{Zr}$ becomes virtually identical, and as a result, both have similar properties. This is because the electrons in the f-subshell do not have a good shielding effect.