I formatted a brand new sandisk usb to ext4 in ubuntu 18.04.2. I have been alternating its use between ubuntu 18.04.3, 64-bit, and debian 10 / armbian, 32-bit. I have been adding files to it until today. 'fdisk -l' is telling me that it is FAT formatted. I am not able to add anymore files.
When I initially formatted it with 'mkfs.ext 4 /dev/sda1', it would not allow me to add any files to it. So I right-clicked on the usb icon in file manager and formatted it by choosing the ext4 format listed there. When I was using the usb in ubuntu originally, I saw a graphical rep of how much space was left, and it showed that it "WAS" ext4.