since n ? 4 as smallest d subshell is 3d, note that the noble gas configuration is usually larger (in the terms i defined above) than the electronic configuration that u specified, in terms of quantum number and subshell (4s2 4p6 vs 3d4 4s0 for example).
i think 'n' was not meant to take the same value in both the electronic configurations. The atomic radius of Na+1
ion is 0.95 A and that of Cu+1
is also the same. I think what was meant is the book is, for two ions of the same size and charge, one with a pseudo noble gas configuration will be more polarising than the cation with a noble gas configuration. So I suppose, CuCl is more covalent than NaCl.