Assault 4 is a gross misdemeanor punishable by up to 364 days in jail and a $5000 fine.
If you have received a subpoena to appear in court you have to show up or risk the prosecutor issuing a material witness warrant for your arrest to compel your appearance. While this is quite rare, it could happen. What is more likely to happen is the prosecutor will either proceed without you based on other evidence or dismiss the case. Its hard to assess the likelyhood of either option without knowing the exact facts of the case.
At trial, you will be asked by the prosecutor and his defense attorney questions about the trial.
Your role is that of a witness. If he is convicted, you will allowed to speak prior to his being sentenced and tell the court what you wish regarding the defendant and sentence in this case.
I have no idea whether the case is good or not since I know none of the facts of the case. You should talk to the prosecutor and the domestic violence victims advocate about the case to get their opinion about the case. They will also be able to tell you what will be able to help with the case.
This is tragic. I am so sorry to hear about your situation. As to what to expect at trial, just take your time and answer the questions that are asked of you. It isn't your job to put a case together. It's the prosecutor's job to put the case together. Tell the prosecutor what you have and then let them do their job. You'll want to give the medical records to the prosecutor (or victim's advocate) long before the trial because it may change the charge and if they want to get the records admitted they'll likely need a witness from the medical facility.
At your service,