The short answer is this is possible but there is no simple setting that you change and it just works. It takes a lot of effort.
You can't do this directly because ext4 is not a valid format for an initial ramdisk. You must convert the filesystem prior to boot. Mount the Raspbian root partition on another Linux system and then create a CPIO archive of all the files on it. The command to do this on Ubuntu 18.04 would look something like this:
proot -0 -q qemu-arm -w / -r /mnt/root sh -c "cd / && find * -xdev -not \( \
-path host-rootfs -prune \
-path run -prune \
-path proc -prune \
-path sys -prune \
-path boot -prune \
\) | cpio --create -H newc" | xz -C crc32 -9 > initramfs
Then you put the resulting archive in the /boot partition of your SD card and specify it as the initramfs in config.txt.
The potential performance benefits of doing this are great. Boot takes longer because it takes a long time to copy the whole filesystem into RAM and decompress it, but once that is done the system boots in about 3 seconds and is very responsive.
However simply converting the default Raspbian image to an initramfs is unlikely to work even with 4GB RAM. The decompression step requires approximately twice as much RAM as the size of the initramfs, and if you get past that the system isn't set up to run from RAM and will likely run out due to temporary files and logs etc.
You also cannot modify the filesystem at all this way. If you want to change a file you have to rebuild the initramfs file on another system.
The best way to do this is to build a completely customized Raspbian image from packages. You can get the size down as low as 50MB for the base system and then add only the things you need.
I have made a tool to do this which you can find at: https://github.com/ali1234/rpi-ramdisk
My tool is capable of building Raspbian ramdisks which can run even on a Pi Zero with 256MB of RAM, and which can also be loaded over PXE or with rpiboot. Unfortunately it is not designed to be easy to use, so if you really want to go full ramdisk you will have to do a lot more research, as there are loads of details I've skipped here for clarity. This answer would have been several pages long otherwise...
Also note that I haven't yet finished adding support for the Pi 4, although it should at least boot.
