Fill in the blanks:
$(1)$ The $IUPAC$ name of $CH_3COCH_2CH_2CH_2COOH$ is .........
$(2)$ The formula of pent$-4-$en$-2-$ol is .........
$(3)$ The $IUPAC$ name of the following structure is ......... and ......... is the main functional group in it.
(Structure: $A$ cyclohexene ring with an $-OH$ group attached to the carbon adjacent to the double bond,such that the double bond is between carbons $2$ and $3$ relative to the $-OH$ at position $1$.)
$(4)$ For $CH_3CH(OH)CH_2CH_2CH_2CHO$,the main functional group and the second priority group are ......... and ......... respectively.