"This trajectory is no linear sequence of "stages" through which we "progress." We do not leave behind an earlier stage in order to advance to the next rung of some hierarchy. All four activities are part of a single continuum of action."I think I understand how this may be. To give an example, I can confront the anguish of one situation and deal with it, but that does not mean I have defeated all possible sources of anguish in my life once and for all. It will reappear again and again in different forms, and every time I will employ each of these four phases Gautama taught. And though it may be possible to engage in one of them without the others, it would be a very unbalanced and ineffective approach. This irreducible quality of the four noble truths is good at combating obsession, so it is nice to see it here. Not merely an article of faith, they are something to be acted upon (and tested first).
One of the difficulties I encountered in understanding this was trying to figure out what the origins of anguish are. I think it is that I don't often enough see the real characteristics of the world, such as transiency, and the inter-dependency of everything, or the need for compassion. Some of the more esoteric schools of Buddhism dwell upon these subjects, and they are an important part of the whole. Essentially, Gautama was motivated by compassion to awaken people to the same realization he had arrived at, and this is the method he used. It's a very positive and affirming approach, and I think it works fairly well.