I have a night flowering cereus that is about five years old. It is a foot tall and has a single stem. It is in a four inch square plastic pot. Is it going to flower anytime soon?
That you have it in a 4" pot and it is a single stem doesn't favor blooming. Assuming this is grown from a cutting it could technically bloom at any time, though the larger the plant the more likely the bloom. Selenicereus are typically vigorous and can flower quickly if they accumulate enough mass. Is yours in a warm sunny enough location? They like their soil moister and richer than the typical cactus though I have found they can thrive in small pots. Try moving it up to a 1 gallon and providing more light to it.