It's impossible to predict the outcome of the newest charge, but I can answer your question, "What's the worst that can happen to him?"
If your boyfriend was on a Deferred Prosecution for his 3rd DUI when he was cited for his 4th DUI, the court can revoke the Deferred Prosecution and impose any sentence between the minimums (either 90 or 120 days if there are two or more convictions in the last 7 years) and the maximum (365 days). The court can also impose a similar sentence on the new DUI (but the statutory maximum is now 364 days).
If the Judgments and Sentence for either of the first two DUIs are still in effect, the court can also impose any or all of the remaining unsuspended jail in those cases.
So, your boyfriend is in it up to his neck. He needs to get an attorney who is familiar with all of the courts where he has pending cases. It will be very important to get agreements with prosecutors as to what they will recommend as punishment AND (hopefully) that all the sentences run concurrent with one another!
The worst that can happen is 364 days in jail and a fine of $5000 plus the possibility of having the prior DUI in the deferred prosecution revoked and the maximum time on that imposed to run consecutively to each other for a total of two years. But it can be worse depending on the details. Are there any cases from DUI 1 and 2 still open? If so, he can do jail time on those as well. There are a lot of possible ramifications including being declared a habitual traffic offender and losing his license for years. Your boyfriend's case sounds like it has some good defenses to it, but he needs to get an attorney working on it as soon as possible.
Since all DUI's can have probation of up to five years he could potential get near a year on each case run back to back. So, the worst that could happen to him is 364 days x 4 minus 1 or 2 days on the first case and 30 or 45 days on the second case.
At your service,
When he gets his 5th DUI within 10 years: and he will. He will be facing a felony DUI charges and prison time. I'm not saying this to be flippant or mean. Even if he beats this charge (and he may be able to based on the facts you outlined) his deferred prosecution will almost certainly be revoked and he will continue to get DUI charges. I've been doing this a long time and I have seen VERY VERY FEW people with this many DUIs get it together. He needs a HUGE reality check or he will spend most of his adult life in jail because he can't or won't stop drinking. What is the worst that can happen? He could get in an accident while drunk and kill you, himself or someone else.
At this point I would focus on what to do to overcome this problem. Hire a good DUI attorney let your counsel take care of creating a good defense. Put your energy into changing the things you actually have control over at this point.
Certainly the other answers are correct; the worst that can happen is that he gets a year in jail for each one, and they run back-to-back.
If you are asking realistically what might happen, an attorney would have to spend some time with the police reports and talk to the witnesses before you could get any meaningful advice. I recommend you talk to someone that practice DUI defense in the court that he is charged in rigth away.
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