Need support and advice...

After much pain and discussion my husband and I have decided we need to put our dog to sleep. He is not well, and is 15 years old, becoming mean and snappish with us at times, among other things. He is just not happy anymore. My 10-yr-old daughter knows of our decision and understands, but how do I deal with it with my 4-year old? He is not especially close to our dog but I am sure he loves him nonetheless. Do we tell him ahead of time? Or do we tell him after the fact, that Wolfie was sick and now he's in heaven? I just want to make it as easy on him as possible. I don't know that he is old enough to really want to "say goodbye" or anything like my daughter might. Has anyone been through this? What did you do? Any help/advice would be so appreciated. A link to my email is on my profile page if you prefer to email me privately rather than leave a comment.