Right next door? Four years? Yeah, chase this guy down until you find him!!
First, put a note on the door so he'll see it if he ever comes to check on the property and you don't see him there.
Your county records have the name of who owns it. Find the name online or down at the county building if you have to. Then you contact him one way or another. Put a letter in the mail with a message talking like you're a real human..."Hey, just curious if you'd be willing to sell that house. We want to buy it."
Even better, call him or direct message him. Search online for that address and/or name and you'll find phone numbers. Social media, whitepages.com, spokeo, homemetry.com, truepeoplesearch.com. Whatever you need to track them down and have a conversation.
Something to consider, based on how you feel about this approach. Call the city code office if you see an offense...tall grass, trash in the yard, etc. Getting a code violation notice from the city may increase the motivation to sell.