Can someone please help me with some code please?

I am trying to create a event macro every time the
workbook is activated and if data is changed from cells A1
thru A4. I want a macro to check these range of cells and
call on another macro when a certain condition is met. In
this case I have the following data

Column A

95
93
null
null

I want the macro to look at the most bottom data in the
four cells and call on a macro if cell A2 is grater than
cell A1.

Now in time data will be entered into cell A3, so when
when data is present in cell A3 I want the macro to be
called if cell A3 is greater than cell A2.

I know this is a little confusing, so any help would be
awesome.

Thanks