Short answer: no, you can't.
Longer answer: mount
/dev/sda4 on one mount point, and do a soft link from the other mount point. Or mount it on a third, application-neutral point, and soft link from both the application points.
Edit: re a tutorial, try:
mount /dev/sda4 /mnt
ln -s /mnt/mysql /var/lib/mysql
ln -s /mnt/xml /home/user/domain/public_html/xml
NB: it is necessary that neither
/home/user/domain/public_html/xml exist, or the
ln -s will do something predictable but unexpected.
Edit 2: it's OK that that stuff exists, you'll need to move it to one side. Having mounted
/mnt (see above), try
mv /var/lib/mysql /var/lib/mysql.mark
mv /home/user/domain/public_html/xml /home/user/domain/public_html/xml.mark
(do the soft links, as above)
mv /var/lib/mysql.mark/* /var/lib/mysql
mv /home/user/domain/public_html/xml.mark/* /home/user/domain/public_html/xml
which should leave you with two empty
.mark directories that can now be removed. Do not do this while the applications are running!