[TYPO3] Upgrading to Typo3 v. 4 - Big problems

[Rune Tallaksen]

Last week I upgraded one of my Typo3 installations from v 3.8.1 to v 4.0
with no problems at all. Tonight I was going to do the same, uploaded the
Typo4 source distro with the following error when hitting
typo3/install/index.php:

Warning:
main(/home/2/i/mydomain/www/tp/typo3/ext/install/mod/class.tx_install.php):
failed to open stream: No such file or directory in
/home/2/i/mydomain/www/tp/typo3/init.php on line 223

Fatal error: main(): Failed opening required
'/home/2/i/mydomain/www/tp/typo3/ext/install/mod/class.tx_install.php'
(include_path='.:/usr/local/lib/php') in
/home/2/i/mydomain/www/tp/typo3/init.php on line 223

What causes this and how to fix it? 
I have extensively tried to search this list and the net with no results
that led me to a solution.

Your help are as always highly appreciated!

Regards, Rune
Norway