You're right, I'd consider that a decomposition reaction is primarily a single species breaking into other species. The IUPAC Gold Book definition for decomposition didn't seem particularity illuminating on this point.

The molecular formula $\ce{C4H9NH2}$ could be a number of different aliphatic primary amines, but all of them are stable under ordinary conditions. In other words, you'd have to "burn" them in a combustion reaction to get $\ce{CO2}$ and $\ce{H2O}$. In such a reaction I would absolutely not expect to get $\ce{N2}$ as a product. Rather I'd expect some sort of nitrogen oxide $\ce{NO_x}$. Which one would depend on temperature, pressure, and the proportion of oxygen.

So the unbalanced reaction would better be written as: $$\ce{O2 + C4H9NH2 ->[\Delta] CO2 + H2O + NO}_x$$

You're right, I'd consider that a decomposition reaction is primarily a single chemical species breaking into other chemical species. Here is the IUPAC Gold Book definition for decomposition.

The IUPAC definition specifically mentions two phases, which implies that the decomposition is "irreversible." In other words, there isn't a significant backwards reaction.

The molecular formula $\ce{C4H9NH2}$ could be a number of different aliphatic primary amines, but all of them are stable under ordinary conditions. In other words, you'd have to "burn" them in a combustion reaction with oxygen to get $\ce{CO2}$ and $\ce{H2O}$. In such a reaction I would absolutely not expect to get $\ce{N2}$ as a product. Rather I'd expect some sort of nitrogen oxide $\ce{NO_x}$. Which one would depend on temperature, pressure, and the proportion of oxygen.

So the unbalanced reaction would better be written as: $$\ce{O2 + C4H9NH2 ->[\Delta] CO2 + H2O + NO}_x$$