Através do artigo no dev net Joan Lafferty, peguei uma listagem contendo a diferença entre alguns componentes do Flex e e do Flex 4.

Vejamos a tabela abaixo:

FLEX 3 MX COMPONENT	FLEX 4 BETA SPARK COMPONENT
mx.controls.Button	spark.components.Button
mx.controls.ButtonBar	spark.components.ButtonBar
mx.controls.CheckBox	spark.components.CheckBox
mx.controls.ComboBox	spark.components.DropDownList (w/o editability)
mx.controls.HorizontalList	spark.components.List (with a HorizontalLayout)
mx.controls.HRule	spark.primitives.Line
mx.controls.HScrollBar	spark.components.HScrollBar
mx.controls.HSlider	spark.components.HSlider
mx.controls.Image	spark.primitives.BitmapImage
mx.controls.LinkBar	spark.components.ButtonBar (with a custom skin)
mx.controls.LinkButton	spark.components.Button (with a custom skin)
mx.controls.List	spark.components.List
mx.controls.NumericStepper	spark.components.NumericStepper
mx.controls.RadioButton	spark.components.RadioButton
mx.controls.RadioButtonGroup	spark.components.RadioButtonGroup
mx.controls.TextArea	spark.components.TextArea
mx.controls.TextInput	spark.components.TextInput
mx.controls.TileList	spark.components.List (with a TileLayout)
mx.controls.ToggleButtonBar	spark.components.ButtonBar
mx.controls.VideoDisplay	spark.components.VideoPlayer
mx.controls.VRule	spark.primitives.Line
mx.controls.VScrollBar	spark.components.VScrollBar
mx.controls.VSlider	spark.components.VSlider
mx.core.Application	spark.components.Application
mx.core.Window	spark.components.Window
mx.core.WindowedApplication	spark.components.WindowedApplication
mx.containers.ApplicationControlBar	spark.components.Application (with the controlBarContent
mx.containers.Canvas	spark.components.Group
mx.containers.ControlBar	spark.components.Panel (with the controlBarContent property)
mx.containers.HBox	spark.components.HGroup
mx.containers.Panel	spark.components.Panel
mx.containers.Tile	spark.components.Group (with a TileLayout)
mx.containers.VBox	spark.components.VGroup

Agora temos as classes que não mudaram nas duas versões:

FLEX 3 CLASSES WITH NO DIRECT FLEX 4 BETA COUNTERPART
mx.controls.Alert
mx.controls.ColorPicker
mx.controls.DataGrid
mx.controls.DateChooser
mx.controls.DateField
mx.controls.Menu
mx.controls.MenuBar
mx.controls.PopUpButton
mx.controls.PopUpMenuButton
mx.controls.ProgressBar
mx.controls.RichTextEditor
mx.controls.TabBar
mx.controls.Tree
mx.containers.Accordion
mx.containers.DividedBox
mx.containers.Form
mx.containers.Grid
mx.containers.TabNavigator
mx.containers.TitleWindow
mx.containers.ViewStack

Agora as diferenças ou chamamos de melhorias nos componentes de texto no Flex 4

	MULTI-LINE	TRUNCATION	SELECTABLE
mx.controls.Label	no	yes	yes
mx.controls.Text	yes	no	yes
spark.components.Label	yes	yes	no
spark.primitives.RichEditableText	yes	yes	no
spark.primitives.RichText	yes	no	yes

	EDITABLE	HTML FORMATTING	SCROLLING
mx.controls.Label	no	yes	no
mx.controls.Text	no	yes	no
spark.components.Label	no	no	no
spark.primitives.RichEditableText	no	yes	no
spark.primitives.RichText	yes	yes	yes

Uma ultima dica, você pode adicionar argumentos na compilação como no Flex 3 e um dos argumentos seria o: 

 Recomendo que vejam o artigo da Joan, pois o mesmo está bem mais detalhado. Veja o artigo clicando aqui

Obrigado a todos.