Perimeter of the nth figure by rule

Unit squares with a side length of $1\,\text{cm}$ are joined together without gaps or overlaps following the pattern shown below. What is the perimeter of the fourth figure, in $\text{cm}$?

Figure description: Small squares, each with a side length of $1\,\text{cm}$, are joined edge to edge to build larger and larger square shapes. The first figure is a single small square ($1\times1$), the second figure is a $2\times2$ square made of $4$ small squares, and the third figure is a $3\times3$ square made of $9$ small squares; the pattern continues in the same way ($\cdots$).