mkfs.ext4 command not found in Debian (Jessie)

Do you have /sbin in your path?

Most likely you are trying to run mkfs.ext4 as a normal user.

Unless you've added it yourself (e.g. in ~/.bashrc or /etc/profile etc), root has /sbin and /usr/sbin in $PATH, but normal users don't by default.

Try running it from a root shell (e.g. after sudo -i) or as:

sudo mkfs.ext4 -L hdd_misha /dev/sdb1

BTW, normal users usually don't have the necessary permissions to use mkfsto format a partition (although they can format a disk-image file that they own - e.g. for use with FUSE or in a VM with, say, VirtualBox).

Formatting a partition requires root privs unless someone has seriously messed up the block device permissions in /dev.