First, it cannot be emphasized enough that you must use that ticket thru a turnstile first, before doing any upgrade. That will bring the value of your ticket up to current, 2012
ticket prices. Otherwise, any upgrade you do, you will have to pay the difference in ticket value from 2006-2012.
At today's ticket price (excluding tax), your 8-day MYW PH ticket is worth $275 + $55 (PH) = $330. You plan to use 4 days and add non-expire option to save the remaining 4 days. Disney ticket prices are front-loaded. The first few days cost the most. After Day 4, extra days only cost $8. Your cost for the first 4 days of that ticket is $243 + $55 (PH) = $298. If you just let the last 4 days expire, you have lost $32. If you buy the non-expire option for those last 4 days, you spend $195 (they charge for the full 8 days of your ticket, not how many days are left).
IMO, I would just use the first 4 days of the ticket and let it expire. OR, upgrade the ticket to a 10-day ticket ($8 + $8 = $16) and add the non-expire option to that (10-day = $225). Then you would have 6 days remaining for one or two future trips.
It was suggested that you add the Water Park Fun & More option if you are considering adding non-expire. This is a great idea, because on your 8 day ticket, you would have 8 of these entries that would never expire.