what function to use to calculate a value based on drop down list

hi all,

I am seeking for help what function shall I use to be able to calculate total value based on selected drop down list e.g.

Cell A4 has drop down list Hrs/MD
Cell B2 has value on Hrs =$22
Cell C2 has value on MD = $176
Cell D4 has value in units = e.g 10
cell E4 = total value base on selected drop down list

E.g. I select Hrs in A4, D4 has 10 Units, E4 = $220
E.g. I select MD in A4, D4 has 10 units, E4 = $1760

thanks for the help
H.