Modding a 4D Maglite - Batteries in series + parallel?

Hi all,

I own quite a few budget lights, but I'm new to modifying torches.

Anyway I've got a 4D maglite which I plan on upgradinging with one of these -

I'm looking at the battery options, I would prefer to use Li-ion, either 18650's or a larger size such as 26500

Since the drop in has a rated max voltage of 8.4 V, and the torch body can hold up to 4 batteries, the "ideal" solution would appear to be 4 batteries, one pair in series, wired in parallel with another pair in series, giving 8.4V max and twice the runtime of a single pair in series?

I had a look round the net to see if anyone else had done this but couldn't find anything much. Obviously there must be a reason for this, is it the safety aspect or are there other reasons why this isn't a good idea?