[jsr]

Are all the settings in the ESC and radio the same? Changing throttle curves, punch, etc will change runtimes.

Also are you driving the same? The buggy might be more controllable now in 6s so you're on throttle sooner/harder than with 4s thus consuming more average current.

there are other factors to runtime to consider. But if truly nothing chnaged then that's odd. But look at all variables first.