I'd bring it up sooner rather than later into your visit (after hellos but before any fancy, romantic anniversary dinners, if possible).
Tell him frankly but calmly how exactly you came across the information (be prepared for some defensiveness, even if you came across it honestly), and then say that while you aren't accusing him of anything, you'd really like to know his motivation for downloading apps for dating.
Don't get me wrong: In your shoes, I would be upset, but I think it's possible you can get through this.
Here's how I think you should do it…First of all, I think you're right to not have this conversation over the phone.
I'm scared to argue over the phone about this so I investigated my leads and found his profile to be deactivated but that still doesn't necessarily demonstrate his intentions on those sites.
I'll be seeing him in less than a week for the first time in months to celebrate our four-year anniversary, and I don't know what to do and how I should address it assuming I even mention it.
Recently, I found out that he had downloaded apps on his phone (Tinder and How About We) sometime after I moved away.
I assure you I didn't find out in a dishonorable way; it was purely an accident that I wish I could forget.