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It is some weird regulation. Really annoying. I bought the light for £30 so I'm very tempted to remove the batteries and hack in new ones. And a better mount. Unfortunately I don't know enough about electronics.
Would a higher capacity but same voltage Li-ion battery pack work?
TRA-
The 'no effort' way to get the advantages of lithium into your light (lighter weight being the main one, as self-discharge rate isn't really a problem for something you use daily), there are these. Without adding some kind of voltage conversion, it's hard to replace a quad pack of NiMh with LiPo since the cell voltage of LiPo is 3.7V, so one cell is probably too little to replace a 4.8V NiMh stack but two is way too much. If you were an experimentalist, it would be worth trying a single cell because the driver circuit in your light probably contains some kind of voltage conversion anyway, and it may have enough input tolerance to live with only 75% of the nominal design battery voltage. The other problem is that LiPo cells tend to be packaged in candy bar shapes, which don't lend themselves to integration into the space previously occupied by a quad of AA cells.
gbj_tester
This should be the case for any StVZO approved light. Pity Philips gave up after getting the optics right and sent it out with a NiMH battery and wonky mechanics. I think the battery is a regulatory problem, StVZO have some weird rules about that.