Lavender – don’t ask him about the divorce. I think that’s rude. Wait for him to bring it up. A fourth date may be too early to expect him to bring it up.
I agree with Trust789 that you should have looked into this before going out. But it’s not too late – you can speak to references at any point. You probably won’t have time before the 4rth date, but as soon as possible, you should call his references or other people who might know about the divorce.
It’s generally best to speak to a Rav who was involved with the divorce. You do want to try to make sure to speak to people who know the story, are objective and who will be careful not to unneccesarily shmear the wife.
One Rosh Yeshiva whom I once called to ask about a divorce just told me that I have nothing to worry about. He didn’t want to speak unnecessary LH about the ex-wife. I was very impressed by this.
It is very unfortunate that when divorced people are in shidduchim, it becomes necessary for everyone to be talking about them, so one has to be careful about hilchos LH and not saying anything unnecessary. At the same time, it is VERY IMPORTANT to check things out as thoroughly as possible.
But you don’t want to speak to people who are just going to keep shmearing the wife unneccessarily (I remember one phone call where I had no choice but to remove the phone from my ears until the person finished talking).
Of course, since you are going on a fourth date, it is more relevant for you to have more information at this point than if you were going on a first date, so all of this is less of an issue, and you should be checkiing things out pretty thorougly.