Let us evaluate the statements:

1. Due to the inert pair effect, the stability of the lower +1 oxidation state progressively increases down Group 13 (Al < Ga < In < Tl). This statement is correct.
2. The acidic character of oxides decreases down the group. $B_2O_3$ is acidic, $Al_2O_3$ and $Ga_2O_3$ are amphoteric, while $In_2O_3$ and $Tl_2O_3$ are basic. This statement is correct.
3. In the complex ion $[Al(H_2O)_6]^{3+}$, the central Aluminium ion ($Al^{3+}$) uses one $3s$, three $3p$, and two outer $3d$ orbitals for bonding, resulting in an outer orbital complex with $sp^3d^2$ hybridisation. The $d^2sp^3$ hybridisation requires inner d-orbitals ($2d$), which do not exist. Thus, this statement is incorrect.
4. Boron naturally exists as two isotopes: $^{10}B$ (approx. 19%) and $^{11}B$ (approx. 81%). This statement is correct.