Because 4.2a SR-4 contains newer code than 4.3 Beta 2, you cannot upgrade from SR-4 to Beta 2.
(You can do it, in that the installer will complete because SR-4's version number is lower than Beta 2's version number, but the program will crash at startup due to changes made in the local data files by SR-4. This will be resolved when 4.3 Beta 3 is released with the latest code.)