今のコードはこんな感じです。

Public Sub シートの書式を設定する()
Dim iRow As Long, iCol As Long, LastRow As Long, ws As Worksheet, r As Range
Application.ScreenUpdating = False
Application.EnableEvents = False
Set ws = ThisWorkbook.Worksheets("対象シート")
startRow = 4
LastRow = ws.Cells(Rows.Count, "B").End(xlUp).Row
For iRow = startRow To LastRow
For iCol = 4 To 100
Set r = ws.Cells(iRow, iCol)
If IsNumeric(r.Value) Then
If r.Value > 0 Then
r.Select
ws.Cells(1, "C").Copy
r.PasteSpecial Paste:=xlPasteFormats
DoEvents
ElseIf r.Value < 0 Then
r.Select
ws.Cells(2, "C").Copy
r.PasteSpecial Paste:=xlPasteFormats
DoEvents
End If
End If
Next iCol
Next iRow
Application.ScreenUpdating = True
Application.EnableEvents = True
End Sub