Are 400 and 45 correct for CD and VL in roman,respectively? can VL and CD be roman numerals for 45 and 400 respectively? By the way,I already tried CCCC for 400.
$400$ would be $CD$ ($500-100$), the same way $4$ is $IV$ ($5-1$)

$45$ is $XLV$ ($(50-10)+5$).

$CCCC$ would probably be understandable (it has been shown that some less educated romans used $VIIII$ for nine instead of $IX$), but it is not the appropriate way to write it.

Roman numbers from 1 to 500
Yes CD is the Roman numeral for 400 but for 45 it's XLV