Here in the states, in municipal precincts, we can expect they’ll have some phone cracking software on hand, so if your TPM is backdoored, your PIN isn’t going to matter much. If yours is an early phone (notoriously the iPhone 5, I think) that doesn’t have a TPM, then it might be susceptible to exploits that lift the limits of tries, in which case a four digit PIN can be cracked by a machine using brute force.