The code is four different digits from 0 to 9, so there are 5,040 possible codes, and you get eight guesses. That sounds tight, but a simple system gets most codes in five or six.
Count the total markers (● plus ○) after each guess. That's how many of the code's digits are among 0–3 and among 4–7. Whatever is missing from four must come from 8 and 9. After two guesses you know roughly where the digits live without having guessed a single one right.
Suppose 0123 scored 2 markers and 4567 scored 1, so one of 8 and 9 is in the code. A third guess that mixes halves, like 0145, tells you which members of each half are in: compare its marker count with what you'd expect.
Once you know the four digits, only the order is left: 24 possibilities at most. A ● means right place. Keep one digit fixed and move the others: if the ● count drops, the digit you moved was in the right spot.
Before you submit, check your guess against every earlier result. If 0123 scored ●○, a new guess containing none of 0, 1, 2, 3 can't be the code. Guesses that could be the answer win games; guesses that only gather information use up turns.
Keep a little list of digits ruled in and ruled out. Most lost games come from forgetting an earlier clue, not from bad luck.