[[1,2]]+[[3,4]]

Let's evaluate: $\left[\right[1,2\left]\right]+\left[\right[3,4\left]\right]$

ⅰ $\left[\right[1,2\left]\right]+\left[\right[3,4\left]\right]$ = $\left[\right[1+3,2+4\left]\right]$

$\left[\right[1+3,2+4\left]\right]$

ⓘ Start from $1$ and move forward $3$ on the number line: $1+3$ = $4$■Start from $2$ and move forward $4$ on the number line: $2+4$ = $6$

▶Result:   $\left[\right[4,6\left]\right]$