Rules page 21 say 'A failure event may occur when you move INTO a square of special terrain'. Later that same page, there's an example where a boneworm gets pushed out of special terrain into regular terrain (it get's pushed off a cliff) and it suffers a failure event.
Obstruction/Cliffs are not placed on a square, but on the edge between squares instead, like doors. Both squares are considered special terrain, but only when figure enters through the edge where the cliff is located.
When the Boneworm(3) is pushed out into the greyish square, it crosses the cliff icon, and therefore suffers the failure event. Same would happen when pushed into square(3) from the greyish square.
But you can freely move from (4) to (3) without penalties.