To run kde4 in a chroot and save your kde3!  Run the follow as root:

Create a mount point:
mkdir /home/chroot

Run debootstrap:
debootstrap sid /home/chroot http://ftp.us.debian.org/debian

Create a testuser:
adduser kde4

Backup /etc/schroot/schroot.conf:

cp /etc/schroot/schroot.conf /etc/schroot/schroot.conf.old

Then replace whats in /etc/schroot/schroot.conf with the following:

# schroot chroot definitions.
# See schroot.conf(5) for complete documentation of the file format.
#
# Please take note that you should not add untrusted users to
# root-groups, because they will essentially have full root access
# to your system.  They will only have root access inside the chroot,
# but that's enough to cause malicious damage.
#
# The following lines are examples only.  Uncomment and alter them to
# customise schroot for your needs, or create a new entry from scratch.
#
#
#[sid]
#description=Debian sid (unstable)
#location=/srv/chroot/sid
#priority=3
#users=rleigh
#groups=sbuild
#root-groups=root
#aliases=unstable,default
#
#[sid-snap]
#type=lvm-snapshot
#description=Debian sid snapshot
##location=/srv/chroot/sid
#priority=3
#groups=sbuild,root
#root-users=rleigh
#root-groups=root,sbuild
#source-root-users=rleigh
#device=/dev/hda_vg/sid_chroot
#mount-options=-o atime,sync,user_xattr
#lvm-snapshot-options=--size 2G
#run-setup-scripts=true
#run-exec-scripts=true
#
#[etch]
#description=Debian etch (testing) 32-bit
#location=/srv/chroot/etch
#priority=3
#groups=sbuild-security
#aliases=testing
#run-setup-scripts=false
#run-exec-scripts=false
#personality=linux32
#
#[sarge]
#description=Debian sarge (stable)
#location=/srv/chroot/sarge
#priority=2
#groups=sbuild
#aliases=stable
#
#[sarge-file]
#description=Debian sarge (stable)
#file=/srv/chroot/sarge.tar.gz
#priority=2
#groups=sbuild
#
#[sarge-secure]
#description=Debian sarge (stable)
#location=/srv/chroot/sarge
#priority=2
#groups=sbuild-security
#aliases=stable-security
#
[experimental]
type=directory
description=Debian experimental (unstable)
priority=4
groups=sbuild,root
users=kde4
#root-groups=root,sbuild
aliases=unstable,default
#device=/dev/hda_vg/experimental_chroot
mount-options=-o atime,sync,user_xattr
location=/home/chroot
run-setup-scripts=true
run-exec-scripts=true

Then backup /etc/shroot/mount-defaults:

cp /etc/schroot/mount-defaults /etc/schroot/mount-defaults

Relace /etc/schroot/mount-defaults with the following:

# mount.defaults: static file system information for chroots.
# Note that the mount point will be prefixed by the chroot path
# (CHROOT_PATH)
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    defaults        0       0
/dev            /dev            none    rw,bind         0       0
/dev/pts        /dev/pts        none    rw,bind         0       0
tmpfs           /dev/shm        tmpfs   defaults        0       0
/home           /home           none    rw,bind         0       0
/tmp            /tmp            none    rw,bind         0       0

Switch to kde4 user:
su - kde4

Then schroot and type root passwd:

schroot -u root

Edit /etc/apt/sources.list and add:

deb http://ftp.us.debian.org/debian/ experimental main non-free contrib

Set up pinning by editing /etc/apt/preferences and adding:

 Package: *
 Pin: release a=experimental
 Pin-Priority: 800
 
 Package: *
 Pin: release a=unstable
 Pin-Priority: 400

Then run update and install kde4:

aptitude update && aptitude install kde4 xorg
  
Then edit /etc/kde4/kdm/kdmrc change StaticServers and ReserveServers:
StaticServers=:1
ReserveServers=:2,:3

Outside of the schroot install sux as root:
apt-get install sux

To login to a full kde4 session run the following:

sux - kde4
schroot
su (use root passwd here)
invoke-rc.d kdm start 

And now you should see kdm and should be able to login as kde4 to kde4!