
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 function $f(x)$ with domain $I$, if there exists an open interval $(a,b) \subseteq I$ and a positive constant $c$, such that for any $x \in (a,b)$, $-c < f(x) < c$ holds, and for any $x \notin (a,b)$, $|f(x)|=c$ always holds, then $f(x)$ is called a "Z-type" function on interval $I$. Consider the following functions:  
1. $f(x)= \begin{cases} 2, & x\leqslant 1 \\ 4-2x, & 1 < x < 3 \\ -2, & x\geqslant 3 \end{cases}$;  
2. $f(x)= \begin{cases} \sqrt {x}, & x\geqslant 0 \\ 0, & x < 0 \end{cases}$;  
3. $f(x)=|\sin x|$;  
4. $f(x)=x+\cos x$,  

Among these, the function(s) that is/are "Z-type" function(s) on interval $I$ is/are __________ (Only need to write the number(s)).

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