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Can anyone explain to me who is Galvatron in TF4?

they said Galvatron was once the Megatron; upgraded.but Megatron already died in TF Dark of the moon

They're talking about the Galvatron from the original cartoon series, aka Generation 1. In the very first Movie, Megatron, from the original series of 1984, gets upgraded into Galvatron, the Decepticon Leader for Season 3, 1986.

That's right, Transformers has been in existence since 1984. Reinvented plenty of times since then:

1984-1990: Generation 1
1992-1995: Generation 2
1996-1999: Beast Wars
2000-2001: Beast Machines

2001-2002: Robots in Disguise

2003-2004: Armada
2004-2005: Energon
2005-2006: Cybertron

2007: Live Action Movie
2009: Revenge of the Fallen
2011: Dark of the Moon

2008: Animated

2012: Prime

Because of those reinventions, there have been several Optimus Primes, Megatrons, and the like. It's no continuous series by any means, it's like Marvel's multiverse, with different versions of each character.