We present a simple, but efficient, way to calculate connection matrices between sets of independent local solutions, defined at two neighbouring singular points, of Fuchsian differential equations of quite large orders, such as those found for the third and fourth contribution (χ⁽³⁾ and χ⁽⁴⁾) to the magnetic susceptibility of the square lattice Ising model. We deduce all the critical behaviours of the solutions χ⁽³⁾ and χ⁽⁴⁾, as well as the asymptotic behaviour of the coefficients in the corresponding series expansions. We confirm that the newly found quadratic singularities of the Fuchsian ODE associated with χ⁽³⁾ are not singularities of the particular solution χ⁽³⁾ itself. We use the previous connection matrices to get the exact expressions of all the monodromy matrices of the Fuchsian differential equation for χ⁽³⁾ (and χ⁽⁴⁾) expressed in the same basis of solutions. These monodromy matrices are the generators of the differential Galois group of the Fuchsian differential equations for χ⁽³⁾ (and χ⁽⁴⁾), whose analysis is just sketched here. As far as the physics implications of the solutions are concerned, we find challenging qualitative differences when comparing the corrections to scaling for the full susceptibility χ at high temperature (respectively low temperature) and the first two terms χ⁽¹⁾ and χ⁽³⁾ (respectively χ⁽²⁾ and χ⁽⁴⁾).