Yes, an 8 cell pack will last twice as long as a 4 cell pack, assuming it's configured in parallel (same output voltage as the 4 cell pack).
However, there is not much benefit to an 8 cell pack over a 4 cell pack + 4 spare cells carried separately. When the light runs out, simply swap in 4 fresh AA cells. If you go for regular alkaline cells rather than rechargeable NiMH ones (such as eneloop), you can always pick them up at a conbini as long as you're not in the middle of nowhere.
Personally I use Panasonic eneloop NiMH cells for all devices that use AA (#3) or AAA (#4) cells such as my helmet light and my backup rear light on brevets. They can be reused hundreds of times and end up costing a fraction of the price of alkaline cells.