After you have accumulated three (3) OWI, BAC or other related and applicable offenses, then you BAC limit goes from a 0.08% to 0.02%. So, if you are pulled over and cited for a 4th offense, it will be based upon a BAC limit of 0.02.
There are sometimes ways around this. For examlple, if you didn't have an attorney on your 2nd or 3rd offense, you may be able to get one of the prior convictions suppressed and make therefore turn the 4th offense into a 3rd offense, and, then the BAC level goes back up to 0.08%.
It is important to gave an attorney who knows how to accomplish this! So, pick your attorney carefully.