Spirit,

To bypass v5 and update your v4 library using v5.0.1 try following:-
-Move 'iPhoto Library' out of Pictures folder to Desktop
-Install and launch v5, this will create a new 'iPhoto Library'
-Update v5 to 5.0.1 and launch
-Quit v5.0.1 replace new 'iPhoto Library' with a COPY (hold down option key when dragging folder) of your old v4 iPhoto folder back into Pictures folder (assuming you have some HD space to do this)
-Launch v5.0.1, iPhoto should detect old v4 library and ask to upgrade it
-Make snack and wait (will take a while)

I haven't tested this but cannot see why it wouldn't work. If it doesn't work you have a copy of your v4 iPhoto Library' sitting on your Desktop — so simple case of reinstalling v4 and moving 'iPhoto Library' back into Pictures folder. Remember, use a COPY COPY COPY!