Consequences of sibling keys when xpub and xprv are leaked
You have interpreted that correctly. If your child extended private key belonging to path m/44'/60'/4'/0 is leaked, the attacker cannot figure out the child extended private key from path m/44'/60'/4'/1.

The non-hardened derivation for child private key is kchild = kpar + hash(Kpar, cpar, i). Here k represents private key, K is the public key, c is the chain code and i is the index number. If you note, the index number sits inside the hash function in the child key derivation. Since hash functions are undeterministic, slight change in its input can have a very different effect on the output. Hence incrementing the index number i from 0 to 1 will yield completely different result and hence cannot be back-calculated.

However, do note that according to BIP 44 derivation, the index number after the account level denotes whether it is receiving or change. The child extended private key that has been leaked (index = 0) will act as a parent private key to derive all receiving addresses belonging to account 5. Thus all receiving addressees derived from the 5th accounts are compromised in your case.