Step 1: Recall the formula for equivalent weight.
For an oxidizing or reducing agent,
\[
\text{Equivalent Weight}
=
\frac{\text{Molecular Weight}}{n\text{-factor}}
\]
where \(n\)-factor is the number of electrons gained or lost per molecule.
Step 2: Determine the \(n\)-factor of \(KMnO_4\) in acidic medium.
In acidic medium,
\[
MnO_4^- \rightarrow Mn^{2+}
\]
The oxidation state of Mn changes from
\[
+7 \rightarrow +2
\]
Thus, electrons gained:
\[
n=5
\]
Therefore,
\[
\text{Equivalent weight of } KMnO_4
=
\frac{M_A}{5}
\]
Step 3: Determine the \(n\)-factor of \(K_2Cr_2O_7\) in acidic medium.
In acidic medium,
\[
Cr_2O_7^{2-}\rightarrow 2Cr^{3+}
\]
The oxidation state of Cr changes from
\[
+6 \rightarrow +3
\]
Each chromium gains
\[
3
\]
electrons.
Since there are two chromium atoms,
\[
n=2\times3=6
\]
Therefore,
\[
\text{Equivalent weight of } K_2Cr_2O_7
=
\frac{M_B}{6}
\]
Step 4: Final conclusion.
Hence,
\[
\boxed{
\text{Equivalent weights}
=
\frac{M_A}{5},\ \frac{M_B}{6}
}
\]
Therefore, the correct option is
\[
\boxed{(4)}
\]