Can do it by hand, at least for this simple case:
>> syms K1 K2 K real
>> syms s
>> K = sym(6);
>> G(s) = K1*K2/(s^2 - 3*s -4);
>> H1(s) = G/(1+G*K)
H1(s) =
(K1*K2)/(((6*K1*K2)/(- s^2 + 3*s + 4) - 1)*(- s^2 + 3*s + 4))
>> H1(s) = simplify(H1(s))
H1(s) =
-(K1*K2)/(- s^2 + 3*s - 6*K1*K2 + 4)
I'm not sure why the SMT prefers to put a negative sign on the numerator and on the leading coefficient of the denominator. It proved surprisingly difficult to change that. Also, check this comment for an approach to convert the result into a tf object after you substitute values for K1 and K2.