Are you uncompressing the image first? The file on the download server is a gzip (.gz) which must be uncompressed before flashing. The command below pipelines the unzipped output into dd.
gzip -cd VERSION-rpi-4-squashfs-factory.img.gz | dd of=/dev/sdX bs=2M
or (advanced usage) you can pipeline from wget direct from the server through gzip then to your card:
wget -O - https://IMAGEURL | gzip -cd | dd of=/dev/sdX bs=2M