How to format a FAT32 USB as EXT4
Solution 1:
You can try to change the partition type in fdisk.
To do this, run fdisk /dev/sda with t you can change your partition type in fdisk, which will lead you through the process. Your partition number should be 1 (because of /dev/sda1) and you should choose the partition type 83 (Linux).
Then try again to run mkfs.ext4 /dev/sda1.
EDIT: To have a better check for your fs type, run df -T which shows you the correct type of each partition.