Have you had a go at installing yet ? If not.....

"Hi.  I was looking at that but still am confused.

-David"

 Lol, not suprised !   

"What is this update script you mention?

• David" 

The update ( Initialise is the correct name , sorry ! ) script appears when you install V4.1 or 4.2. An alias to it gets put on your desktop but you can move that to somewhere else. It runs some sort of script  that is necessary when you add the Morphs++, Creature or Muscle Morphs to the V4 base to get them working .

" I only have V4.0.  If I install 4.2 will that mess everything up?
-David

 What "everything"  do you mean ? I believe it will overwrite V.4.
Are you wanting to keep the original V4 you have incase you don't like V4.2 ? 
( Though Daz does say in that 25 page post that they DO NOT recommend keeping multiple versions of V4 in your Runtime )  

If you want to keep V.4 'just in case' the following steps are what I did incase I wanted to reinstate  V4.1-

1. Simplest is to back up your current Runtime .....
   or at the very least make a copy of the V4 folder that is in the Library>!!Daz folder and as well copy your V4 folder that is in the Library> Character  folder and put them somewhere else outside of Poser. I did the V4 obj too just to be safe.
   Copy and move any V4 Morph packages ( in Pose Folder) you also have installed, ie Morphs++, Muscle or Creature to a folder outside of Poser.
   This way if you don't like V4.2 you can delete her and drop these all back where they came from.

2)   Don't know if you have all these, I do and this is the order Daz says they should be done in :
Run the Victoria_4_2_Update installer 

Run the V4 Morphs installer 
Run the V4 Muscle Morphs installer 
Run the Creature MSV4 installer
Use the initialise script when the installers give you the option to.

3. ONLY IF you have the Creature Morphs already installed with V4 after updating you need to manually remove the following -
   .RuntimelibrariesPoseDAZ's Victoria 4Morph InjectionsINJ Creature V4toV3.png 
   .RuntimelibrariesPoseDAZ's Victoria 4Morph InjectionsINJ Creature V4toV3.pz2 
   .RuntimelibrariesPoseDAZ's Victoria 4Morph InjectionsREM Creature V4toV3.png 
   .RuntimelibrariesPoseDAZ's Victoria 4Morph InjectionsREM Creature V4toV3.pz2 

The BIG change from V4 to V.4.2 is that it's gone back to the INJ system ( which I personally think is a PITA !) so you have to manually load the morphs via Pose>Daz's Victoria 4>Morph injections.
But AprilYSH has kindly made a cr2 that you can use so she loads with the Morphs ++ already in as did V4 and V4.1.  http://aprilsvanity.com/dlpage.php 
Or add them to the V4.2 base yourself and save the figure to your library.

 Hope this helps, and it's not too much or too little. 
The Mac forum here is a good place to ask Mac/Poser questions too.
 V4.2 is working a treat for me , other than the my initial irritation about the INJ stuff .

" Try and be nice to people, avoid eating fat, read a good book every now and then, get some walking in, and try and live together in peace and harmony with people of all creeds and nations."
-Monty Python