Zsh, 94 94 bytes

Edit: Lost 3 bytes to fix a bug (if first key is backspace, a[1]= makes $a an array). Saved 3 bytes by not using %s in the printf statement.

a= while (($#a<4)); do read -sk1 k case $k {$'\C-?')a[1]=;;[0-9])a+=*;} printf '\r\e[K'$a done 

asterisks= # $#foo: length of $foo while (($#asterisks < 4)); do # -s: silent/noecho, -k 1: one keypress read -s -k 1 key case $key { # backspace: delete first character from asterisks string $'\C-?') asterisks[1]= ;; [0-9]) asterisks+=* } # \r: start of line, \e[K: clear to end of line printf '\r\e[K'$asterisks done 

Zsh, 94 87 bytes

Lost 3 bytes to fix a bug (if first key is backspace, a[1]= makes $a an array). Saved 3 bytes by not using %s in the printf statement, then 7 more by switching from while to for.

a= for ((;$#a<4;)) {read -sk1 k case $k {$'\C-?')a[1]=;;[0-9])a+=*;} printf '\r\e[K'$a} 

asterisks= # $#foo: length of $foo for ((;$#asterisks < 4;)); { # -s: silent/noecho, -k 1: one keypress read -s -k 1 key case $key { # backspace: delete first character from asterisks string $'\C-?') asterisks[1]= ;; [0-9]) asterisks+=* } # \r: start of line, \e[K: clear to end of line printf '\r\e[K'$asterisks } 

Zsh, 94 bytes

while (($#a<4)); do read -sk1 k case $k {$'\C-?')a[1]=;;[0-9])a+=*;} printf '\r\e[K%s' $a done 

# $#foo: length of $foo while (($#asterisks < 4)); do # -s: silent/noecho, -k 1: one keypress read -s -k 1 key case $key { # backspace: delete first character from asterisks string $'\C-?') asterisks[1]= ;; [0-9]) asterisks+=* } # \r: start of line, \e[K: clear to end of line printf '\r\e[K%s' $asterisks done 

Zsh, 94 94 bytes

Edit: Lost 3 bytes to fix a bug (if first key is backspace, a[1]= makes $a an array). Saved 3 bytes by not using %s in the printf statement.

a= while (($#a<4)); do read -sk1 k case $k {$'\C-?')a[1]=;;[0-9])a+=*;} printf '\r\e[K'$a done 

asterisks= # $#foo: length of $foo while (($#asterisks < 4)); do # -s: silent/noecho, -k 1: one keypress read -s -k 1 key case $key { # backspace: delete first character from asterisks string $'\C-?') asterisks[1]= ;; [0-9]) asterisks+=* } # \r: start of line, \e[K: clear to end of line printf '\r\e[K'$asterisks done