Increment a cell value - conditions applied

Hello,
I needed a formula to add a value to a cell when the cell next to it contains a value where that value is obtained by incrementing the value in the previous cell by 1.

I used the following formula:

=IF(B4<>"",A3+1,"")

Example.
A3 contains a value of 1.
The formula in A4 monitors cell B4 and if a value is entered in B4 then a value is placed in A4 which equates to A3+1 (ie value of 2).


This works well enough as seen in rows 3-6 above, BUT when the previous cell in column A contains no value - such as cell A in row 7 in the example above, the formula no longer works in cell A8

What I need to do is somehow amend the formula (or use some other method) so that the cell which is incremented in column A is the last one in the column which contained a value.

I hope this all makes sense and wonder if anyone can assist with suggestions for a solution?

TIA