Get a letter from your military enlister saying you have cleared the initial background check and your misdemeanor summary probation is the only thing holding you back from being accepted. Then add the case onto the court calendar by filing a motion to (favorably) terminate probation. Serve the prosecutor with a copy of this motion. Make sure to attach the appropriate documentation.
It's always advisable to hire an attorney for this process.
An attorney would need more information about your facts to make an educated decision. However, it is possible to modify DUI probation, but it's very difficult to do so because the court wants to send a message not to drink and drive. The fact that you completed the classes and paid the fines is important. Is this your first DUI? If so, it would be easier to modify DUI probation. I don't know why you would get four years of probation on a first misdemeanor DUI unless the facts were bad or that's what they do in San Diego. If this is your second DUI or third, I doubt you would be able to modify DUI probation. One thing you would need to do is show that you are serious about getting into the military. Do you have emails from a military recruiter to show your lawyer?
Converting probation to jail is possible. I doubt they would go for community service or sheriff labor. You may need to do straight jail. Jail would be no more than a year, but you do get jail credits for good behavior to reduce your sentence. You should talk to the public defender or contact a lawyer to discuss these issues. Have all the information from the recruiter to show the lawyer. Then get the case added to calendar to modify the sentence.
Yes this is something that is possible. San Diego will typically give 5 years of probation on all DUI cases so there is nothing unusual there. The issue that happens in these cases is the military is unable to guarantee acceptance and the court does not want to take you off probation without knowledge that you are eligible to enlist in the army. This is something that you need to work with the recruiter to make sure they explain that the probation is the only remaining hurdle for you to overcome in getting into the military.