Not good. A fourth DUI in Michigan while actively on probation for the third is about as bad a combination as it gets in the drunk driving context. The new DUI itself triggers a new criminal prosecution, and the probation violation from the third offense runs alongside it. He is now looking at two separate legal matters hitting at the same time.
In Michigan, a fourth DUI is a felony under the habitual offender provisions that apply after multiple convictions. Felony DUI carries potential state prison time, not just county jail. The fact that he committed it while under court supervision for the previous offense is exactly the kind of circumstance that tells a judge the prior leniency did not work, and courts respond to that by removing the leniency.
The honest assessment is that he is very likely looking at some meaningful period of incarceration this time. Whether that is enough to change his relationship with alcohol is a different question entirely, and the answer to that one lives inside him rather than inside a cell. Substance abuse treatment would be the most valuable thing to come out of this process, but that requires him to want it. The court may mandate it, which at least puts the option in front of him whether he wants it or not.