Delete Rows to reduce file size Macro

I'm currently using the code below to delete rows that are not needed for our intended metrics. When I dump the raw data (up to 6k rows), there is a column that has null data up to exceeding row 8100 in column D. Therefore, the code below only delete the rows below 8100 (or whatever row the null data ends up to any particular day). What I want is to have the rows deleted below the starting empty cell in column B... So if in column B the data ends in row 5223, than delate rows below that whereas reducing the size of the sheet and file... Unless there is another better way to do this without deleting the relevant data rows... thx



With ActiveSheet
Range(.Cells(.Rows.Count, 4).End(xlUp).Offset(1,0), .Rows(.Rows.Count)).Delete
End With