Artificial intelligence (AI) investing has been a key market theme over the past two and a half years, and it's slated to remain a significant part of investing for the next few years as well. The reality is that massive AI infrastructure is still needed, and the build-out isn't slated to slow anytime soon.Even though many stocks have already risen significantly, it doesn't mean their upside is capped at today's levels. I wouldn't be surprised to see these four continue to generate substantial returns for i ...