Can anyone tell me if a hard reset of a discovery 4 would take off engine management light? Would I just remove negative lead for 20mins or so? Thanks

No it won't. The MIL is on for a reason. The only way to find the reason is with a diagnostic reader.

The D4 is a very complex vehicle, and so shouldn't be played with by someone randomly disconnecting the battery (there are normally 2 anyway), so do yourself a favour and take the vehicle to someone qualified and equipped to run a proper diagnostic on it.