Another feature, which also helped improve the range of the -400 series, is that it has fuel tanks added on the horizontal stabilizer. It basically has fuel anywhere where it fits. The horizontal stab. can hold 3,300 gallons of fuel.
The increased wingspan, as mentioned above, enabled an additional leading edge flap section making a total of 11 sections on each side (not including the Kruegers on the inboard sides)
Rudder deflection is increased to +/- 30 degrees on the -400
Wheel diameter was increased by 2 inches. This enlargement enables the undercarriages to house BFGoodrich carbon brakes. The previous versions of the 747 were heavy maintainance on the undercarriages apparently!
The APU is changed to a Pratt APU, the PW901A, which proved fuel savings of up to 40%. I am not sure which was the previous APU on the 747s.
On the interior, the -400 has a "glass cockpit", and restyled interiors. One of the big mistakes boeing made was to offer a huge amount of possibilities for the customers to choose configuration options on the interior design. Sometimes delivering the aircraft required very long additional working hours to cope with customer needs.
Those are most of the changes made on the -400 compared to the -300, including those mentioned above. Hope it helps