It will definitely fit, but there is a risk that the springs will be too soft for a 400 (8-cylinder) engine...
The 400E is heavier as the 300E and will therefore need stiffer front springs. The rear springs will not matter that much, they will probably be equal.
I have mounted C200D/C220D (4-cylinder) Billstein/Eibach springs under my C250D (5-cylinder). It worked out fine and I have driven approx. 250.000km on these lowering springs by now without any problem.
It depends on the weight difference whether you can use 300E springs on a 400E...Does anyone know what difference in weight there is?
with a minor 600+k