Chemical equation that can be balanced in infinite number of ways?
We first note that your reaction (4) can be broken down into two reactions by splitting up those chemical species that depend on $n$ and those that don't: $$\begin{align}\ce{CH4 + H2O &amp;-&gt; CO + 3H2}\\
\ce{CO2 + H2 &amp;-&gt; CO + H2O},
\end{align}$$
so your reactions (1) through (3) are just statements about the relative rates of these two reactions.


  Which one is true? If all of them are true, why my textbook mentions only one of them?


I presume none of them are really true, in the sense that the ratio of the reaction rates isn't something as nice as 1:1 or 1:2 or 1:3, etc., and we haven't accounted for side reactions either. But assuming a 1:1 ratio is certainly a useful simplification.


  What really happens in reality?


The ratio of the rates will depend on the temperature and pressure and composition of the feed. There will be side reactions that occur. You can perform experimental measurements of the product composition to determine what happens.


  Is equation 2 just a simplification of more complicated equations?


Yes.


  Does this have to do anything with the reaction mechanism?


No. The overall reaction does not, in general, tell you anything about the reaction mechanism.