It was a lot sooner for me than 4 years, it was only about 3 months when I asked her during a phone call whether it was worthwhile me waiting for us to get back together again. She said 'No'. So it was like a switch being flicked. I moved on past there.
Mind you, your ex-wife has more or less said the same thing to you by actually getting the divorce already.
Face it. It's over. She doesn't (can't !!) matter to you any more. Turn the page. Burn your bridges. Step out the back, Jack. I was 42 when I had to start again. Let me tell you that old saying is correct: "Life begins at 40!."
You need to work on your lowish self-esteem. It's what's telling you that there can't be anybody else after her. But yes there can.
Read up on some books regarding how couples split up. A good one for me (but 30 years ago) was a book called 'Passages' by Gail Sheehy, which explained what mental and emotional changes occur in people as they get older and go through the various stepping-stones of life. I was able to see why my wife left me too. And also to understand that the marriage break-up wasn't all my fault but was due to both of us.
Oh, my story? Almost exactly 12 months later I moved in with a woman who I later married. We've been together now for 34 years.