I signed two forms. One said that I owed them a fifth year of active service for the free E-4. The second said that I owed them two months of active duty for every month of training that they gave me, including A school, up to a maximum of six years active duty. If you make it through NPS and prototype, you will owe them the whole six years whether you take the promotion or not and whether you graduate or not.
And no, you do not owe them eight years. The basic enlistment period is six years. For most people, that is 4 active and 2 inactive reserve. For some, it is 2 active and 4 active reserve. For nukes it is the former, with the addition of up to 2 years active duty as stated above. If you do the six, without reenlisting, you are totally free. If you do at least four but less than six, the balance of the six years is inactive reserve.