Well, by definition, a brute force attack would mean to try out all possible passwords.
You have unlimited tries, so that's a great start. Only numbers (0-9) are allowed, and a fixed length of 4 characters? That's excellent. Now you only need 10,000 tries.
While this is an extremely easy password, it is unfortunately on an old device, so you could not automate the cracking process.
You will need to manually try passwords like so:
0000, 0001, 0002, 0003, ... until the password is accepted.
In the worst case, it will take you 10,000 tries.
Now, let's say it takes about 10 seconds to try one password because you need 3 seconds to enter the digits, and the phone says "wrong" and freezes for like 7 seconds or something. So that is 100,000 seconds to try all the possibilities, which is about 27 hours. You can perhaps split this task into 10 days, so each day you'll spend 10,000 (a little under 3 hours) seconds to try the password. It's probably a good idea to turn on some radio or watch some TV while trying in case you get really bored of it.