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.