What processor do I have?
How can I tell if my processor is a 32bit or 64bit? How do I know if I can upgrade it to 64bit if it is 32bit? So much to do, so little time. All comments appreciated.
Here’s the Intel page about the i5-4570S chip.
That iMac has a chip capable of processing a 64-bit Instruction Set, running a 64-bit Operating System (Mac OS X 10.8.4 (12E4022) or newer with that iMac), which will run 64-bit applications (and currently also 32-bit applications).
As the next version of macOS is expected to step away from 32-bit applications, you may want to check which of your current applications are 32-bit, and seek updates for those you want to keep using with the next major version of macOS.
32-bit app compatibility with macOS High Sierra 10.13.4 - Apple Support
The warnings you may have gotten are about (old) applications, and their compatibility with future operating systems. It is not (directly) about the CPU chip, and you can’t change really change the chip, as the whole motherboard design is tied to the current chip.
To see which of your applications are 32-bit, go to menu > About This Mac > System Report > Software > Applications. Then click on the 64-bit column to sort the Yes’s and No’s. Ask the developers of the ‘No’ software (=32-bit) that you would want to keep using, if they will be releasing 64-bit versions of their software.
