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I would recommend using 6 digits to be safe. We had a gust lecture with Cellebrite, and they actually brute forced them self into iPhone by trying a PIN code, wait and tried another one. The difference between 4 and 6 digits was quite big in the time it took to get access (like weeks compared to almost a year I think). Not sure how iPhone throttle failed attempts (might be a fixed time), but they got in eventually.
4 digit pin gives 5040 combinations (210 if they should be unique), while 6 digit pin gives 151 200 combinations.