Try changing your code to this:
Sub mergeCategoryValues()
Dim lngRow As Long
With ActiveSheet
lngRow = .Cells(65536, 1).End(xlUp).Row
.Cells(1).CurrentRegion.Sort key1:=.Cells(1), Header:=xlYes
Do
If .Cells(lngRow, 1) = .Cells(lngRow - 1, 1) Then
.Cells(lngRow - 1, 3) = .Cells(lngRow - 1, 3) & "; " & .Cells(lngRow, 3)
.Cells(lngRow - 1, 4) = .Cells(lngRow - 1, 4) + .Cells(lngRow, 4)
.Rows(lngRow).Delete
End If
lngRow = lngRow - 1
Loop Until lngRow = 1
End With
End Sub
Tested
EDIT
To make it a little easier to adjust to different column I added variables at the beginning to indicate which column do what. Note that column 2 (B) isn’t used in the current logic.
Sub mergeCategoryValues()
Dim lngRow As Long
With ActiveSheet
Dim columnToMatch As Integer: columnToMatch = 1
Dim columnToConcatenate As Integer: columnToConcatenate = 3
Dim columnToSum As Integer: columnToSum = 4
lngRow = .Cells(65536, columnToMatch).End(xlUp).Row
.Cells(columnToMatch).CurrentRegion.Sort key1:=.Cells(columnToMatch), Header:=xlYes
Do
If .Cells(lngRow, columnToMatch) = .Cells(lngRow - 1, columnToMatch) Then
.Cells(lngRow - 1, columnToConcatenate) = .Cells(lngRow - 1, columnToConcatenate) & "; " & .Cells(lngRow, columnToConcatenate)
.Cells(lngRow - 1, columnToSum) = .Cells(lngRow - 1, columnToSum) + .Cells(lngRow, columnToSum)
.Rows(lngRow).Delete
End If
lngRow = lngRow - 1
Loop Until lngRow = 1
End With
End Sub