How many numbers lie between the largest $$3$$-digit number and the smallest $$4$$-digit number ?
Largest $$3$$-digit number = $$ 999 $$
Smallest $$4$$-digit number = $$ 1000 $$
Required number= $$ (1000 - 999) $$
= $$1$$
Therefore only $$1$$ number lies between the largest $$3$$-digit number and the smallest $$4$$-digit number.