Is there something I'm missing here? I can't choose not to trigger it. If I keep my LR Hired Guns clumped up it may back fire, or my opponent may choose to kill the first one in the squad off in order to deplete last resort, then kill off the adjacent one that no longer has last resort.

Am I interpreting this correctly? Maybe I shouldn't have 4-5-6 of these attachments in my squad, haha.

It is not mandatory because you have to pay the cost to trigger it.

Also, you cannot use more than 4, because you can only have 4 of any regular deployment card.

All abilities with a cost are voluntary. See Abilities, page 2 of RRG.

Somewhat snarky response:

And apparently all abilities on your and your opponent's cards are also optional. See ridiculously long thread about Agent Blaise.

It is not mandatory because you have to pay the cost to trigger it.

Also, you cannot use more than 4, because you can only have 4 of any regular deployment card.

I thought that attachments don't count as deployment cards?

"Deplete" is a cost. Costs are not mandatory. When the trigger occurs, you may choose to spend the cost or not.

It is not mandatory because you have to pay the cost to trigger it.

Also, you cannot use more than 4, because you can only have 4 of any regular deployment card.

I thought that attachments don't count as deployment cards?

They still count as deployment cards for the purposes of deployment points and army building.

"Choosing Deployment Cards", SG, Page 6:

Name Restriction: Each army is limited to a number of Deployment cards with the same name as follows:

-- Maximum of 1 of each unique Deployment card. These cards are identified by a bullet before the name (for example, “• Darth Vader”).

-- Maximum of 2 of each elite (red) Deployment card.

-- Maximum of 4 of each regular (gray) Deployment card.

• Skirmish Upgrade Cards: Some Deployment cards have the “Skirmish Upgrade” trait. These cards provide special abilities, but unlike normal Deployment cards, they do not have corresponding figures. These cost deployment points and often have a game changing ability (for example, allowing an army to contain Deployment cards from multiple affiliations).

I thought that attachments don't count as deployment cards?

These are not the deployment cards, you are searching for, when reading the Temporal Alliance Card. For any other purpose, they are.

Not quite any other purpose. Abilities do not affect Skirmish Upgrade Deployment cards unless specifically specified. For example, Take Initiative cannot exhaust a Skirmish Upgrade card, whether an Attachment or not.

From the FAQ:

"Some Deployment cards have the Skirmish Upgrade trait. Rules
and abilities that reference Deployment cards do not affect, do not
include, and cannot manipulate Skirmish Upgrade cards unless
the rule or ability states otherwise."