I just had a similar need of accessing Dropbox (to see what I have there before I remove the account) and booted my old work laptop to do this.
It’s encrypted and not using ext4… I however ended up picking a different solution that allowed me to still run their client. I simply made a new 10 GB file, formatted it as ext4 and mounted over loopback and pointed Dropbox at the mountpoint.
Just takes a few steps:
dd if=/dev/zero of=~/dropbox.img bs=1M count=10240
sudo mount -o loop ~/dropbox.img ~/Dropbox
sudo chown -R $USER:$USER ~/Dropbox
and just start up Dropbox, the changes are transparent for it.