What qualifies as an “automatic loadout weapon”

There are no skulls on Halo 4 (similarly to Reach, they are unlocked from the beginning).
However, here are the names and what they do from this source. I hope this is useful.

The details for each of the skulls in Halo 4 can be seen below:

Famine - Weapons drop much less ammo.

Tilt - Enemy resistances and weaknesses are increased.

Mythic - Enemies have increased health.

Catch - Enemies are grenade happy!

Black Eye - Shields don't recharge unless you melee enemies.

Tough Luck - Enemies always go berserk, always dive out of the way, and never flee.

Iron - Co-op: Back to previous checkpoint on death.

Solo: Restart mission on death.

Thunderstorm - Major upgrade to the capabilities of enemies.

Cloud - Motion sensor is disabled.

Cowbell - Acceleration from explosions is increased.

IWHBYD - Rare combat dialogue becomes more common.

Grunt Birthday Party - Headshot a Grunt, HAPPY BOOM TIME!

Blind - The HUD and the first person arms and weapon are hidden.

Frag grenades are on a timer, and explode after several seconds, regardless of the surfaces they contact or where they roll. The advantage of the frag is that you can bounce it off of surfaces, and therefore you can do indirect damage to enemies that are around a corner. You can also get a bit more distance between you and a target, as they will bounce and roll on flat terrain.

Plasma grenades stick to surfaces, and then detonate shortly afterwards. This can make them good against enemies that tend to dodge when grenades are thrown at them. If you connect with the throw, it's a guaranteed hit, and in most cases a guaranteed kill. However, without the ability to bounce, they're a bit shorter range and can't be used as effectively for indirect fire.

Pulse grenades are the new Promethean grenade type. When thrown, they detonate when the hit a surface. However, they will spend several seconds creating a shield- and health- draining sphere with a fairly limited radius. It's about as tall as a Spartan, Elite, or Knight. After a couple of seconds, they explode, doing significant damage. Since they're a bit more area-of-effect and damage-over-time, they're good for when you know you have an enemy pinned or stunned. The sphere's area of effect also slows vehicles caught within it.