granted i only did this with 2 NICs, but the process should be the same. note the RHEL link at the bottom of the page. that talks about the correct location you have listed for the bonding.conf. yes you are correct it is in /etc/modprobe.d/foo.conf
yes, you create your ifcfg-bond# as master, then your eth0-3 will be slaves, or in your case em1-4.
good luck, post back with settings and results, id love to see how you did this as well as your performance.
*side note* question, what kind of switch are you using? i just had a conversation last night with a RH engineer. he pointed out that unless you are using a high end managed switch, you will most likely see a degradation in performance not an increase, especially for mode 4 and 6.