Something like this
Public Class Form3
Inherits System.Windows.Forms.Form
'Snip
Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim frm As New Form4
frm.ShowDialog()
DoWhatYouWantWith(frm.Selection)
End Sub
End Class
Public Class Form4
Inherits System.Windows.Forms.Form
'Snip
Public ReadOnly Property Selection() As Something
Get
Return WhateverTheUserSelected
End Get
End Property
End Class
And if all you want to do is select a file to open or a location to save, there's a component for that. Scroll down on the Windows Forms category on the toolbox and look at the <blankity blank>Dialog controls. These actually provide a good example of the pattern above in action. For example, OpenFileDialog has a property name Filename that is set to what the user is selected. You might do something like:
If OpenFileDialog1.ShowDialog = DialogResult.OK Then
Dim fs As New IO.FileStream(OpenFileDialog1.FileName, IO.FileMode.Open)
'Do something
fs.Close()
End If
Notice that ShowDialog returns a DialogResult. There's a property called DialogResult on the Button class. So for an OK button, set it to DialogResult.OK so you can easily discern what button the user pressed (For example if they clicked cancel you don't want to do anything when ShowDialog returns)