the same code as mentioned by WayneRyan with an addition of the following
Select Case Me.YourCombo
Case "TextA"
Me.Field1.Enabled = False
Me.Field2.Enabled = False
Me.Field3.Enabled = True
Me.Field4.Enabled = True
Me.Field1.Value = 0
Me.Field2.Value = 0
Me.Field3.Enabled = WhatEverValueYouWant
Me.Field4.Enabled = WhatEverValueYouWant
End Select