Would it be wise to make another round of culls choosing the birds that are laying now? I am down to 4 pullets. How long of a delay from the first pullet laying to the last pullet laying is acceptable?
If you have culled down to your four best pullets, I would go with my four best pullets. There will be enough variability in the offspring (in your flock) to not do any lasting damage. Then if they are laying five eggs a week during laying season, four would be twenty, and two would be ten. It easier to select from qty. than not. Sometimes we start with a trio, and it forces us to hatch many small batches over a length of time.
POL is an average, so yes, there is a range. I think the commercial guys figure it in % laying at a certain point. Not when the first egg is identified. 20-22 wks. is a very good range for this breed. Decide what is too late for you, and do not keep any past that point. That is a strength of theirs at this point, so all you need to do is maintain it, but you do need to maintain it.
There are advantages to them reaching POL a little later, so do not feel inclined to push them. Let good management and genetics dictate when.
24wks is a reasonable average for most American Breeds, but the Delaware and New Hampshire were noted for their fast feathering, their rate of maturity, and their early fleshing. We should keep an eye on what their strengths were. They were noted to be in the 20wk range, but 22 is very good. Why not enjoy the advantages the breed should offer? Let the 28-32 wk layers be enjoyed by others with other breeds that are not selected to be any different than what they already are.
There are some color challenges with this breed. Of course that is not your priority at this point, but there are points you do not want to neglect all together either. You have heard build the barn before you paint it, and it is a good saying. BUT, you do not want to re build the barn to paint it. That would not make a lot of sense. Hold on to what you have, and be mindful of the specifics of this color variety. The color variety of the Delaware is a unique characteristic of the breed. There are none marked like it. It is an actual color to learn to breed. Well marked birds do not happen on their own, and they do not maintain their color on their own.
Vigor and Health 1st. Type and productivity 2nd. Color is last, but that point is often overemphasized to the point of neglect. That is a mistake. Learning to breed the color is part of learning to breed a breed. The finer points can wait, and there is plenty to worry about now, but we would not want to lose what we have. It is not hard to do, but it is hard to get back.
As you move into growing out your first generation, it begins to be the work of your hands, and their will be even more to consider. Fortunately, there will be more to select from, and plenty of time to watch and evaluate them.