Thanks, Ruchi, now I understand a bit :)
If you Install 32-bit now, you would be able to use 3.21GB of RAM out of 4GB, as this is a design limitation. For 4 GB, we recommend 64 bit though.
If you still want to switch to 32-bit, use Rufus to create an Installation media, and in that, select MBR partition scheme for UEFI/BIOS. https://www.tenforums.com/tutorials/2376-create-bootable-usb-flash-drive-install-windows-10-a.html
In the BIOS menu, enable Legacy boot, disable UEFI, boot from the installation media, and install Windows. Please note that if currently the hard drive is configured as GPT, you need to format it. To check if the drive is configured as GPT or MBR, you can find it in the disk management>Right-click the drive>Properties.