This may be your first achievement. Once you get to Part 1, Chapter 2, head to the coin lockers and talk to the guy with the blue arrow above his head to get the Coin Lockers 1 memo. He will also give you the Locker Key Finder. Equip that, then run around Kamurocho listening for the familiar chime sound from past games. When you hear it, stop and look around for the key. You know your getting close when the sound gets faster. Next, examine it to pick it up. When you have 4 more, return to the coin lockers (red and white mural on the left) and open them for the pop.
Note: You can also use guides written by CyricZ from Gamefaqs:
Taihei Blvd.: https://gamefaqs.gamespot.com/ps4/239456-yakuza-4-remastered...Underground: https://gamefaqs.gamespot.com/ps4/239456-yakuza-4-remastered...