Apps are compiled Java, they're not subject to the bitsize of the CPU. As long as the ROM you're running supports Android apps (and that can now even be a Chrome browser running on a Windows computer), an Android app will run.
What 64 bits will give us is slightly more speed (due to the fact that data is transferred 64 bits at a time instead of 32 bits at a time) and the number of bits supports a memory address up to 16 extabytes or 16X10^[sup]18[/sup] bytes (which, of course, we'll probably never see in a single computer because, due to the speed of light, reaching the physically farthest RAM from the CPU would take so long that the CPU would have to be incredibly slow). But we probably will see phones with 4, 6 or 8 GB of RAM in the not too distant future.
That doesn't mean that an app written to run on a Gingerbread phone won't be able to run on a 64 bit phone. I have plenty of programs I wrote for DOS - running 16 bit CPUs - that still run on 64 bit Windows computers in a 64 bit cmd window. And that's an executable running directly in the CPU. Android apps run on a virtual Java machine, so as long as there's a Java machine for the CPU, and it retains the older APIs (and there's no reason not to - Windows 8.1 still retains all the applicable DOS 1.0 APIs), the app will run.
Look at the amount of internal storage (is it enough for all the apps you want to install? Don't ever depend on running apps from the external SD card) and look at the internal RAM size (which can't be larger than 4GB - the size of the display in a 32 bit machine). I don't think anyone is going to write apps needing more than 3GB of RAM in the next 2, 3 or 4 years. (And even if you're a software junkie, as I am, the Note 4 should be enough phone for you. Without cleaning out all the trash [I have a few email apps, a few launchers, a few browsers, etc., that I haven't uninstalled since I decided on which ones I want to keep] I still have over 7GB of internal app space free.) I think I'll be keeping my Note 3, and running the latest apps on it, for a lot more than 2 years.