Use button to scroll down Richtextbox 4 lines

Question

I have a RichTextBox in vb.net which shows data from a .txt file using the "ReadAllText" methode.

As this program is running on a device with a touchscreen, i want to use "big", real buttons to scroll up and down rather than a scrollbar.

So i made a "scroll up" and "scroll down" button. The scroll down button is supposed to scroll down 4 lines from the last visible one. So if the Textbox looks like this:

1
2
3
4
5
6
7
8
9
10

i am seeing

1
2
3
4
5

due to the size of the textbox and i click the "scroll down" button, it should now scroll down 3 lines. So i should be seeing

5
6
7
8
9

however i can't seem to make it work like this.

This is what my code looks like currently:

        Dim lastVisibleLine As Integer = RichTextBox1.GetLineFromCharIndex(RichTextBox1.GetCharIndexFromPosition(New Point(0, RichTextBox1.Height)))

        Dim scrollToLine As Integer = lastVisibleLine + 4


        Dim charIndex As Integer = RichTextBox1.GetFirstCharIndexFromLine(scrollToLine)
        If charIndex >= 0 Then
            RichTextBox1.SelectionStart = charIndex
            RichTextBox1.ScrollToCaret()
        End If

however if i currently see lines 1-5 in the textbox, this makes it jump straight to 10-15.

Can anyone help me?