I looked briefly into getting one of these.
The ES4 is the ES2 with the extra battery included/preinstalled. The slowing down when the headlight is on thing,... is what they do. Something to do with how the motherboard is wired. Poor engineering with how the light gets its power, perhaps.
Leave the light off and use a headlamp on your head is a possible solution. Otherwise, there are people on other forums who know how to re-solder the motherboard to avoid the problem.
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