TurboWarp, 424 bytes

Try it here.

Key-presses are really hard to manage well.

NOTE: This will not work in Scratch, because backspace is only detectable in TurboWarp.

NOTE: In the scratchblocks version and the picture, I used a custom block containing 1 argument. This was because I had to actually use the block rather than leave it as a function, and scratchblocks needs something in a custom block to recognize it as one. The project linked to will use an empty block, however.

edit: translating into scratchblocks is HARD

define( set[i v]to(0 repeat until<(i)>(9 if<key(i)pressed?>then set[x v]to(join(x)[* stop[this script v] end change[i v]by(1 end when gf clicked set[x v]to[ repeat until<(length of(x))=(4 if<<key(any v)pressed?>and<not<key(backspace v)pressed?>>>then ( end if<key(backspace v)pressed?>then set[j v]to((length of(x))-(2 set[x v]to[ repeat until<(length of(x))>(j set[x v]to(join(x)[* end end wait until<not<key(any v)pressed? 

TurboWarp, 424 423 bytes

Try it here.

Key-presses are really hard to manage well.

NOTE: This will not work in Scratch, because backspace is only detectable in TurboWarp.

NOTE: In the scratchblocks version and the picture, I used a custom block containing 1 argument. This was because I had to actually use the block rather than leave it as a function, and scratchblocks needs something in a custom block to recognize it as one. The project linked to will use an empty block, however.

edit: translating into scratchblocks is HARD

define( set[i v]to(0 repeat until<(i)>(9 if<key(i)pressed?>then set[x v]to(join(x)[* stop[this script v end change[i v]by(1 end when gf clicked set[x v]to[ repeat until<(length of(x))=(4 if<<key(any v)pressed?>and<not<key(backspace v)pressed?>>>then ( end if<key(backspace v)pressed?>then set[j v]to((length of(x))-(2 set[x v]to[ repeat until<(length of(x))>(j set[x v]to(join(x)[* end end wait until<not<key(any v)pressed? 

TurboWarp, 426 bytes

Try it here.

Key-presses are really hard to manage well.

NOTE: This will not work in Scratch, because backspace is only detectable in TurboWarp.

NOTE: In the scratchblocks version and the picture, I used a custom block containing 1 argument. This was because I had to actually use the block rather than leave it as a function, and scratchblocks needs something in a custom block to recognize it as one. The project linked to will use an empty block, however.

define( set[i v]to(0 repeat until<(i)>(9 if<key(i v)pressed?>then set[x v]to(join(x)[* stop[this script v] end change[i v]by(1 end when gf clicked set[x v]to[ repeat until<(length of (x))=(4 if<<key(any v)pressed?>and<not<key(backspace v)pressed?>>>then ( end if<key(backspace v)pressed?>then set[j v]to((length of(x))-(2 set[x v]to[ repeat until<(length of(x))>(j set[x v]to(join(x)[* end end wait until<not<key(any v)pressed? 

TurboWarp, 424 bytes

Try it here.

Key-presses are really hard to manage well.

NOTE: This will not work in Scratch, because backspace is only detectable in TurboWarp.

NOTE: In the scratchblocks version and the picture, I used a custom block containing 1 argument. This was because I had to actually use the block rather than leave it as a function, and scratchblocks needs something in a custom block to recognize it as one. The project linked to will use an empty block, however.

edit: translating into scratchblocks is HARD

define( set[i v]to(0 repeat until<(i)>(9 if<key(i)pressed?>then set[x v]to(join(x)[* stop[this script v] end change[i v]by(1 end when gf clicked set[x v]to[ repeat until<(length of(x))=(4 if<<key(any v)pressed?>and<not<key(backspace v)pressed?>>>then ( end if<key(backspace v)pressed?>then set[j v]to((length of(x))-(2 set[x v]to[ repeat until<(length of(x))>(j set[x v]to(join(x)[* end end wait until<not<key(any v)pressed?