Snooping is wrong, very wrong. That said, I have done it. I have even done it around someone's recovery work, though I don't believe I would have ever dared to look at someone's Fourth Step. The only ones who are ever supposed to hear about the Fourth Step are God and the person's sponsor (or whoever they choose to hear their Fifth Step).
What's done is done. I certainly wouldn't go and tell him that you did that--it might be harmful to his recovery. What you do with whatever distressing thing you have learned is to keep it to yourself, or confide in a therapist or Al-Anon sponsor.
And PLEASE make a vow to yourself that you will stay OUT of his recovery and his private thoughts and papers from now on.