which are true?

$1$When a problem's solution can be verified in a polynomial time it can be classified in the NP complexity class.

$2$If P and NP classes would be the same it doesn't necessarily mean that for all problems in the NP class there would exist polynomial time solutions.

$3$The big$-$O notation in complexity theory stands for an order of the function characterizing computational resources consumption growth rate on an input size growth.

$4$ Decidable questions are such questions for which one can write a program that will always terminate.