Protecting a short password
The adversary has


a ciphertext encrypted under your key;
a copy of the key with $k$ bytes erased; and
knowledge of which $k$ bytes have been erased.


The adversary can try to decrypt the ciphertext and decide if the decryption makes sense in time $T$.

To find the key, the adversary need only do $2^{8k}$ trial decryptions, for a total time cost of $2^{8k} T$.

The key point is that there's usually no sensible way to make $T$ big.

Ergo, when $k$ is $4$, this is obviously easy.