Generally in DO schools you schedule 4th year yourself via VSLO or simply by calling or emailing the places you would like to rotate with. Your school may allow you to do some or all of your 4th year at your 3rd year clinical site.
Whether or not you “have to” or “want to” is specialty and individual dependent. Same for matching in a specific location. If you want to to FM or IM in Illinois, pass all your licensing exams on the first try and it shouldn’t be an issue. If you are looking at more competitive specialties it gets harder to be focused on a single region since you will likely have to apply to a large number of programs.
Try not to worry about it too much right now and focus on getting through year 1! Best of luck