thraldom > slavery abide > remain; wait, await the outcome
3 And the just revolution measured,
just > exact; full, complete measured > [is measured]
4 That they as strangers shall be notified.
strangers > foreigners, outsiders [in their own country] notified > denoted, called
5 For twice four hundred years shall be supplied
twice four hundred years > (Virtually the exact interval between the death of Cadwallader in about 690 and the enthronement of Henry VII in 1485)
6 Ere they to former rule restored shall be, 7 And their importune fates all satisfied:
importune > grievous, burdensome
8 Yet during this their most obscurity,