Is my Raspberry Pi 64-bit?

The short answer:

The Raspberry Pi 3 and 4 are 64-bit compatible, so they can run 32 or 64 bit OSes. 

As of this writing, Raspberry Pi OS 64-bit is in beta: Raspberry Pi OS (64 bit) beta test version, while the 32-bit version (previously named Raspbian) is a stable release.

The long answer:

As previously mentioned above, the Pi 3 and Pi 4 are 64-bit capable, but there is little benefit to use a 64-bit OS until you have more than 4GB of memory (simplified explanation, of course).

With the introduction of the Pi4 8GB, this is no longer the absolute truth: 
"Our default operating system image uses a 32-bit LPAE kernel and a 32-bit userland. This allows multiple processes to share all 8GB of memory, subject to the restriction that no single process can use more than 3GB. For most users this isn’t a serious restriction, particularly since every tab in Chromium gets its own process. Sticking with a 32-bit userland has the benefit that the same image will run on every board from a 2011-era alpha board to today’s shiny new 8GB product." Source: 8GB Raspberry Pi 4 on sale now at $75 - Raspberry Pi Blog, Eben Upton, 2020-05-28. (Eben Upton is responsible for the overall software and hardware architecture of the Raspberry Pi device. Source: Wikipedia, 2020-06-22.)

So, what does it mean?

That a single process, running in Raspberry Pi OS 32-bit/Raspbian, cannot use more than 3GB. However, many programs use a multitude of processes, and EACH process can use up to 3GB (the Chromium navigator is an example, a process for each tab). So, a program that uses many processes CAN use all of the 8GB of memory in the newest Raspberry Pi 4 8GB, even if running under a 32-bit Raspberry Pi OS.