The path difference at the centre is zero.
The path difference at the $4^{th}$ bright fringe $= 4\lambda$
Here, $\lambda = 6000 \times 10^{-10}\, m$
$\therefore$ Path difference $= 4 \times 6000 \times 10^{-10} $
$= 24 \times 10^{-7}$
$= 2.4 \times 10^{-6}\, m$