The primary replacement for the B747-400 passenger version, at the moment, are the different versions of the B777, the A340-500 and the A330. When it arrives the B787 will become a major contender.
The A380 is not a 744 replacement as it only serves a niche market and is unlikely to ever be produced in the same numbers as the B747.
As a freighter the B747F has no equal at the moment, the B747-8F will be an improvement on the -400 but you will probably see the B747-400 around for many years yet in various configurations.