Create a file similar to this one:
srcup:
Code:
*default tag=RELENG_4_5
*default prefix=/usr
*default base=/usr/local/etc/cvsup
*default host=cvsup2.freebsd.org
*default release=cvs delete use-rel-suffix compress
src-all
Then do:
Code:
cvsup -g /path/to/srcup
You may need to make the /usr/local/etc/cvsup directory.
You've now got the source for 4.5 in /usr/src.
As for the kernel thing... are you planning on just changing the kernel but leaving the userland alone (you're going to break things if you are)? Or are you looking to downgrade your complete system?
Either way, I can help but you should understand that there is a difference between replacing just the kernel and not the world. Also, why are you looking to downgrade?