It’s not uncommon for a 2-year-old hybrid to stop laying for a long time. They are not machines, after all. Sometimes they come back into lay, maybe in the next breeding season the following year, but there’s no guarantee.
Of course, in a small flock an unproductive hen is a big drawback, especially when you only have one laying out of the four. It would be expected for the 4-year-old exbatts to have come through the henopause, used up all their egg cells and are now unable to lay. This is good if you want to keep them on as pensioner pets because they can sometimes go on for years,and will be less afflicted by egg laying problems which are so common in middle aged or elderly hybrids, eg prolapse or peritonitis or eggbinding arising from a soft egg getting stuck in the egg passage, Not so good if they are mainly kept for eggs, and you don’t have room for new pullets.
Big decision time, sadly, I think. It depends on how you regard the nature of the ‘deal’ you have with them - whether you decide to keep them on as pets for the rest of their natural life, or whether you feel you have done your best for them whilst they were able to provide the eggs you need, and then you decide to give at least the oldest ones a peaceful end, having given them all that extra time and love which they couldn't have had if they hadn’t found you, and start again with new ones. Never easy, and I’ve tried both ways, though nowadays I tend to go for the second option. Either way is fine, it’s just what sits best with you and what you want and need from your little flock.