Pretty much all modern turbofans run with a blade tip speed at full throttle of mach 1.5 or so.The compressor /burner behind the duct ed fan spin at high rate. The fan must keep the tip of blades below sound speed.
4MW isn't anywhere close to enough. Propulsive power at cruise for a modern jetliner the size of a 787 is more on the order of 40MW. At takeoff, each engine has to be making more like 50-60MW of shaft power just to run the front fan. Rerun the electric numbers knowing that and you'll see why running jetliners on electric power is a pipedream without a massive breakthrough in technology.Latest jet engines have a reducing gear from the compressor shaft to the fan ( just like turboprops).An electric motor will spin the fan ,with practically 100% efficiency, without any gear.A Dreamliner burns 1.3 Kg/sec.That is 50 Mj/sec ( or Mw). My guess is that 4 Mw electric will be needed operating the fan from an electric motor from a battery. weighing 120 Tons (6 hours flight). The plane will burn 28 tons of fuel.280 passengers @60 Kg/passenger is 17 Tons. Conclusions: 1)I am not suggesting to convert a Dreamliner to e drive,2)batteries are no more than an order of magnitude away from intermediate range, subsonic aircraft for passenger flight.