Add Conditional Formatting Criteria With Code

I recorded some code and cleaned it up to apply a conditional format

Sub condi_format_I()
Dim wbBook As Workbook
Dim ws As Worksheet
Dim rng As Range

Set wbBook = ThisWorkbook
Set ws = wbBook.Worksheets("Summary")
Set rng = ws.Range("C4:P52")

With rng
.FormatConditions.Delete
.FormatConditions.Add xlExpression, Formula1:="=C4<0"
.FormatConditions(1).Interior.ColorIndex = 45
End With

End Sub

I would like to apply a custom color index to all cells in range C4:P52 if the value is less than zero.

But when I execute the macro, many cells receive the custom format that do notr match the custom format. Other cells that should receive the custom format do not