By 17670s I meant plural 17670.
A protected lithium ion battery has built-in circuitry that prevents damage catastrophic failures. This includes overcharging, but unless you have a faulty charger, this isn't anything to worry about with unprotected cells, as the chargers themselves have overcharge protection.
The real advantage in protected cells are regarding overdischarge, shorting, and reverse polarity. Putting an unprotected cell in a charger or light can cause serious problems. Overdischarging a li-ion can damage the cell reducing capacity, or even more serious problems. Shorting the battery can also cause serious problems. And by "serious problems" I mean venting (pressure build-up), venting with flame (think of a roman candle), or exploding. In any of these situations, the protection circuitry on a protected cell simply shuts the battery off. No worries at all.
The disadvantage is that in many flashlights, you won't get a warning when your battery is low, and you'll suddenly be left in the dark. There's also a very slight reduced runtime with a protected cell, as the circuitry siphons a bit of juice itself, but IMO it's not enough to worry about, and definitely not worth the risk. There's also some flashlights where an unprotected cell will fit, but a protected cell won't.
For the Surefire L4, stick with protected 17670 cells. Cheaper than the Pilas anyway, and work just as well.