There is no 10.4, just 10.04. The first number is for the year, 2010 and the
second number is for the fourth month, April (04 not 40). This is the year
and month of release. If you change the numbers you destroy the meaning as
10.10 which comes out next October would have a lower number than the April
release and that does not make sense.

You cannot upgrade from 32 bit to 64-bit. You would have to do a fresh
installation. It is easier than you might think and there are benefits to a
fresh installation. You get a clean slate and you avoid problems that often
accompany upgrades often leftover from old kernels and changes in the OS. In
this case it would likely have to do with the removal of HAL and the
inclusion of Plymouth.

You can backup the list of your applications to a text file and then use it
to reinstall applications. However, you would have to re-download them all.