
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given a sequence $\{a_n\}$ with the sum of its first $n$ terms denoted as $S_n$, if $S_n$, $S_{n-1}$, and $S_{n+1}$ (for $n \geqslant 2$) form an arithmetic sequence, and $a_2 = -2$, then $a_4 =$ ______.

Present the answer in LaTex format: \boxed{Your answer}
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