Now this is a discussion on a fun and interesting tactic, and also just looking to see if there are any flaws in it. It should go into the tactics section, but it is neither an army list or a battle report, so I decided to put it here.
Going through the new daemon codex tonight, and comparing it with the old codex noting the changes, I noticed a few things that would make a few units really good at daemoning the quad gun.
All the lesser daemons have a BS skill. Even though they generally do not have any ranged weapons (except for the ones the heralds can get) they can be used to sit behind the Aegis defence line and shoot the quadcannon. Now 3 types of lesser daemons come to the fore here -
Nurglings - they are non scoring troops (because they are swarms) so are normally used as a covering line for other models, which is just the same as their invul save 5+ (unless you are shrouded). If using the old models for them, they sit really low behind the ADL and need 4+ to hit the target with re-rolls, and Line of Sight is measured from the actual weapon, as you can assume the weapons has control panel with monitor and joystick to allow you to shoot with it behind cover, oh and they get shrouded and Defence grenades for being Daemons of Nurgle.
Bloodletters - They stick up behind the ADL, but with a BS of 5 they need a 2+ to hit with re-rolls, and the cover save allows them to survive longer. Any enemy charging over the ADL without assault grenades, will be at a lower init, and they have AP 3 strength 4 weapons (with Hatred for re-rolls) when the enemy decide to assault you lines.
The last type is Plaguebearers who again have only a BS of 3, meaning a 4+ to hit, and shrouded, and defence grenades. They also have 4+ poisoned swords, and a Tgh of 4.
Units like the Helldrake with Bale flamer will have a field day with many of the units used for this type of role, but the invul save the daemons get might allow for a couple of them to survive.
It is something I noticed, but might make your opponent sit up and take notice when your daemons decide to sit back behind cover rather than charge in to your waiting gun line.