I've been wanting to do this for myself, but haven't knuckled down on it yet.
The process should be similar to what you would do if you were upgrading the HDD on your old node.
Under the standard linux install, the blocks are in a ~/.bitcoin subdirectory. I think it's a matter of moving the raw data from that place to whatever your new place is (your new raspy 4).
After moving the block data, my guess is that the new node will need to re-validate the block data.
Would love to hear more about your experience on this, or any links that you find along the way.
~/.bitcoinsubdirectory. I think it's a matter of moving the raw data from that place to whatever your new place is (your new raspy 4).