Equivalence of Secret IV and Secret Prefix MAC
There are a number of assumptions being made here, and I will make them explicit in case this was not what you had in mind.

The first assumption is that we are using a Merkle-Damgard hash. That is, given a compression function $C(h, m)$, we have (assuming a properly padded message)
$$
H_{\text{IV}}(m_0 \| m_1\| \dots \| m_n) = C(C(C(C(\text{IV}, m_0), m_1), \dots), m_n).
$$

The secret-prefix MAC is simply $H_{\text{IV}}(k \| m)$. I also assume that secret-prefix MAC uses the entire first block, that is, the secret key $k$ is the same size as $m_0$ above.

The secret-IV MAC sets the $\text{IV}$ to be the key: $H_{k}(m)$. But notice we can easily convert a secret prefix into a secret IV by setting $k' = C(\text{IV}, k)$. Therefore if you have a key-recovery attack on a secret-IV MAC, the process is exactly the same with secret-prefix, except the output you obtain is $C(\text{IV}, k)$ instead of $k$. Likewise, an attack on secret-prefix MAC is highly likely to target $C(\text{IV}, k)$ and not $k$ directly. To define the IV, you cannot use the standard APIs for hashing (e.g., md4_init(&amp;ctx)), but will likely have to write your own code.

Recovering this secret IV, in either case, will allow you to forge any message and completely destroy the security of the MAC. In the secret-prefix case, however, recovering the IV does not immediately translate to recovering the original key.