All Raspberry Pi 4/Pi 400 boards are 64-bit, however they will report themselves as 32-bit when using the normal Raspberry Pi OS as the standard operating system is 32-bit.
The 64-bit version of the operating system has been in development since the Pi 4 was first released, and is still being worked on by Raspberry Pi to make sure it's as bug-free as possible before releasing it to all users.
This is why Raspberry Pi recommend and ship the 32-bit OS, as it's stable, reliable and has everything the majority of users will need.
Download 64-bit Raspberry Pi OS (Advanced users)
If you do want to try the 64-bit OS - you can! Head to the link below to download the current 64-bit OS image file, then use the Raspberry Pi Imager tool ('Use custom' option) to write it to your micro-SD card.
Remember - this isn't a stable release yet (as of 25/01/2021) so you may come across bugs or performance/stability issues:
Further information, including known bugs can be found here: https://www.raspberrypi.org/forums/viewtopic.php?t=275370