D4: Dark Dreams Don't Die

Dark Shaman 13 found a nice exploit to gain quick credits.

After completing Roland Walken’s case in episode two, complete the game then replay any episode by choosing episode select from the main menu. Go into Young’s bedroom. In here will be a Roland doll, and when you interact with this, you can play the clover touch mini-game in Young’s bedroom.

Keep playing the clover touch game until you get your "MVP Score Hunter" achievement. Once that’s done, you’ll get at least 1,000 credits. The higher the score, the greater your credit payout will be.

Go to the phone in the hallway outside of Young’s bedroom, and save at the phone. Head back into the bedroom and pick up the Roland doll again (press the B button to skip cutscenes). Right when the clover game starts, press B, and give up the game. The game will end, and when it does, the game will still reveal the high score from your last game (like 1,200), and you’ll get another 1,000 credits.

Start the game again, skip the little opening cutscenes, and continue to give up the game. You’ll keep getting those credits. You can also continue to go out to the hallway and save the game every once in a while, but you must not exit the game and come back to the save or else you will have to replay clover touch and set up a high score again.