From ven. Bodhi's footnotes from "In the Buddha's Words" (and further details in the Visuddhimagga ( http://www.accesstoinsight.org/lib/auth ... on2011.pdf
))regarding those 4 paths:
1. Samathapubbaṅgamaṃ vipassanaṃ. Mp: “This refers to a meditator who first obtains serenity and then takes up insight meditation.” The commentators call such a meditator one who makes serenity the vehicle of practice (samathayānika ). See Vism 587; Ppn 18:3.
2. Vipassanāpubbaṅgamaṃ samathaṃ. Mp: “This refers to one who by natural bent first attains to insight and then, based on insight, produces concentration.” In the commentarial literature this is called one who makes insight the vehicle (vipassanāyānika). See Vism 588; Ppn 18:4.
3. Samathavipassanaṃ yuganaddhaṃ. In this mode of practice, one enters the first jhāna and then, after emerging from it, applies insight to that experience, i.e., one sees the five aggregates of the jhāna (form, feeling, perception, etc.) as impermanent, bound up with suffering, and nonself. Then one enters the second jhāna and contemplates it with insight. One applies the same procedure to the other jhānas as well until the path of stream-entry, etc., is realized.
4. Dhammuddhaccaviggahitaṃ mānasaṃ hoti. Mp says that “agitation” (uddhacca) arises here as a reaction to the ten “corruptions of insight” (vipassanūpakkilesa) that one misunderstands as indicating path-attainment. (On the corruptions of insight, see Vism 633–38; Ppn 20:105–28.) It is possible, however, that the “agitation about the teaching” is mental distress brought on by eagerness to realize the Dhamma. This state of spiritual anxiety, when suddenly resolved, can sometimes precipitate an instantaneous experience of awakening. For an example, see the story of Bāhiya Dārucīriya at Ud 1:10