The answer to your first question is simple, check if the PictureBox.Bottom is greater than or equal to the Form's height or bottom. If it is, then set the PictureBox.Location equal to a random x and a constant y:
Code:
Option Strict On
Option Explicit On
Public Class Form1
    'Declare a new instance of the random object here,
    'or declare it here, and set it as a new instance at the form load
    Private r As New Random

    Private Sub check_pb()
        For Each pb As PictureBox In Me.Controls.OfType(Of PictureBox)()
            If pb.Bottom >= Me.Bottom Then
                pb.Location = New Point(r.Next(0, Me.Width), 5)
            End If
        Next
    End Sub


End Class
As for your second question, I'm not sure I understand your question. Is it: if the picturebox hits the bottom 4 times, the game ends? If so, then declare an integer variable and increment it by 1 when you set the PictureBox.Location:
Code:
Option Strict On
Option Explicit On
Public Class Form1
    'Declare a new instance of the random object here,
    'or declare it here, and set it as a new instance at the form load
    Private r As New Random
    Private bottom_hit As Integer = 1

    Private Sub check_pb()
        For Each pb As PictureBox In Me.Controls.OfType(Of PictureBox)()
            If pb.Bottom >= Me.Bottom AndAlso bottom_hit < 3 Then
                pb.Location = New Point(r.Next(0, Me.Width), 5)
                bottom_hit += 1
            ElseIf pb.Bottom >= Me.Bottom AndAlso bottom_hit = 4 Then
                'End the game here

            End If
        Next
    End Sub


End Class