Error check to see if cells are filled in

I have this MICRO that simply selects a cell

Sub Step4()
    Range("A91").Select
End Sub

... but I DO NOT want it to do that IF cells "D4" and/or "J4" and/or "G4 are blank.

Rather return an error message saying:
(If D4 is blank) "Please enter your name."
(If J4 is blank) "Please enter the quantity of your order."
(If G4 is blank) "Please enter a language."

Thanks!