during updating to the new beta I ecountered some problems, because the nightly 1.4-beta2 contains no checksum.inc.php. Because my old version of this file was not overwritten, update refused to continue.
I then deleted the old checksum.inc.php and everything was OK.

I would like to suggest to give the user the possibility to start installation even if checksums are not correct. (Maybe with some extra expaination and warnings?)
I also would like to suggest, that have a look after the line break because I have a hard time reading the warnings.