question about misfire on a 2000 S500

My 2000 s500 with 55k miles (out of warranty) misfires.. That is when I start driving it cold it runs good unless I step on the gas hard.. then is stumbles and doesn't accellerate at all.
I took it to the dealer and they hooked it up to the computer which said it had a misfire in cylinder 4 and 6 and would be about $400 to fix it..
Any idea what this means ? or if I could have a normal mechanic fix it instead of my overcharging dealer?
thanks
Jason
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Misfires upon acceleration usually occur because the air / fuel ratio is too lean. You step on the gas, the throttle plate opens but there's insufficient fuel for all the air that's rushing into the cylinders. Since the dealer kindly told you that #4 and #6 were the offenders I'd have those injectors checked, also their spark plugs and, depending how the wires are organized, their coils.
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