4. Place a three-digit number $m$ in front of a two-digit number $n$ to form a five-digit number. It can be represented as ( ).
(A) $m n$
(B) $m+n$
(C) $10 m+n$
(D) $100 m+n$
4. D

Translate the text above into English, please retain the original text's line breaks and format, and output the translation result directly.