Here comes the "math" question. In the unlikely event this seedless wallet were to get compromised and an attacker gained access to the 4 private keys, how easy (if at all) would it be to generate the private keys to gain access to the original main wallet?
I believe what is required for an "attack" like that is:
- The Master Public Key
- At least one Private Key
As described here:
https://bitcointalk.org/index.php?topic=657205.msg7385160#msg7385160If you just have 4 private keys that get compromised, then there is no way to derive the other keys.
It also depends, to a certain extent, on whether or not the keys are "hardened", as per this Trezor wiki entry:
For security reasons, using hardened keys is safer, but there are use cases for using non-hardened keys. A parent extended public key together with a non-hardened child private key can expose the parent private key. This means that extended public keys must be treated more carefully than regular public keys. It is also the reason for the existence of hardened keys and why they are used for the account level in the tree. This way, a leak of account-specific (or below) private keys never risks compromising the master or other accounts.