As always it is always best to let the user decide, and offer them the option, as when scrolling on a PC, to choose a line scroll (analogous to using the cursor keys) or a page scroll (analogous to PgUp and PgDn).
Each method has its merits:
- Line-by-Line gives the user a less sudden flash to a new environment, and it easier on the eyes.
- Page-by-Page offers a more rapid, flick through the document option. Useful for scanning a document, or returning to the point where the reader left off, during the previous reading
It is always a good idea to give the user reference, as to where the last line was prior to scrolling. So, using your example of a list containing eight items of which only four items are shown at a time, if you were to do a page-by-page scroll then from lines 1-4, then show 4-7, and finally 7-8.
Line scrolling can employ scrolling by half lines to offer a similar smoother effect.
The book, The Essential Guide to User Interface Design: An Introduction to GUI Design By Wilbert O. Galitz offers some good tips regarding scrolling, pages 179-180. Unfortunately I can not paste the quote here, but follow the link.