First, name the cells B1 to B4 :
by selecting B1 to B4 and going to the top left screen and write a
name for them ( ex. LIST ).
Second, select A1 then on the top menu select DATA>VALIDATION
On the setting tab dropdown to LIST
then in the second box write " = LIST" without quotations.
Check "ignore blank" and "in-cell dropdown"
Click OK and try it.