No More Heroes 4 could happen, though series creator Suda51 says that fans would have to be really loud about it to get one.
While No More Heroes 3 was meant to be the last entry in the series, Suda51 hasn't necessarily ruled out another game somewhere down the road, though it sounds like it would be quite a long road if it ever did happen. The director recently spoke with GNN about the most recent game, asking if it's possible we'll see Travis Touchdown once more.
"Yes, that’s right, Heroes No More 3 is Travis’ last battle,” Suda said (translations via VGC). "Nobody could say what’s going to happen 10 years from now, and I can’t say either. If there’s a big fan outcry, Travis might be back. Just like Tom Cruise came back as Maverick."