An observation about 4 digit PIN's. They're even weaker than you might think just from "doing the math" at least in some cases. Sure, there's 10000 combinations to search through if you're trying to brute force one, but I'd bet money that in most cases you don't need to search anywhere near that many.
Case in point: I had a unit at a mini-storage place once. And you needed a 4 digit PIN to get through the gate. And I forgot the PIN I used. I was sitting at the gate for a minute, staring at the keypad and realized "wait... hundreds of people have PINs in this system and the system doesn't care which one you use". So I just needed a PIN that somebody used. So I started with years that would have been reasonable birth years for an average adult at that time and starting going up. I think it took about 6 tries to find a valid PIN.
Now granted, this is different than trying to brute force a specific person's PIN. But even then, I expect that in many cases an informed search will crack it a lot faster than a purely sequential search or a random search. Using common birth years, well known numbers like "5150", "1234", "4321", etc. is probably going to work a lot of times.
Case in point: I had a unit at a mini-storage place once. And you needed a 4 digit PIN to get through the gate. And I forgot the PIN I used. I was sitting at the gate for a minute, staring at the keypad and realized "wait... hundreds of people have PINs in this system and the system doesn't care which one you use". So I just needed a PIN that somebody used. So I started with years that would have been reasonable birth years for an average adult at that time and starting going up. I think it took about 6 tries to find a valid PIN.
Now granted, this is different than trying to brute force a specific person's PIN. But even then, I expect that in many cases an informed search will crack it a lot faster than a purely sequential search or a random search. Using common birth years, well known numbers like "5150", "1234", "4321", etc. is probably going to work a lot of times.