Regarded as a set of four equations in the four unknowns a, b, c and d, (k being a parameter),
the equations will possess non-zero solutions only if the determinant of coefficients is equal to zero.
That is,
11−k0011−k−k0111−k01=0
Expanding, that gets you
k4−2k2−4k=0, or, k(k−2)(k2+2k+2)=0, so k=0 or k=2.
k = 0 produces the solution a = t, b = -t, c = t, d = -t , t a parameter, from which a/b + b/c + c/d + d/a = -4.
k = 2 produces a = b = c = d = t, t a parameter so a/b + b/c + c/d + d/a = 4.
So the maximum would appear to be 4.