About security, only then yours BTC are safe where/when:
1. Your public addres has no output transactions
2. If your public address has no more output transactions than 4.
you ask me why step 2?
for example: if and only if you have more transactions output that n , example : 40 (as outputs - 40 x r,s,z)
you can try implement algorithm - superpositional linear equation - and you need to find only 4 transactions in the same level bit to recalculate private key.
Just to be clear: there are a lot of addresses where the public key is known, that's kind of the idea of a
public key. Furthermore there are plenty of those with even thousands of signed transactions. So was for any of those addresses/public key the private key ever calculated? Plain simple:
No.