TPN means a 4 Pole device with 4th Pole as Neutral.
In TPN opening & closing will open & close the Neutral.
For TPN, protection applies to the current flows through only 3 poles (Three Phase) only; there is no protection for the current flow through the neutral pole. Neutral is just an isolating pole.
TP MCB is used in 3phase 4wire system. It is denoted as TP+N which will mean a three pole device with external neutral link which can be isolated if required.
For the 4 pole breakers, protection applies to current flow through all poles.
However when breaker trips or manually opened, all poles are disconnected. Same type of difference also applies for SPN and DP