Hi I just got the method to continue after elite four:
(before you do it, its recommended to backup the battery and states folder, renaming all save file same with rom file name)

save from in game saving (before ending screen), save state
exit desmume
download converter save file from http://filetrip.net/f21802-UniqueGeeks- … -Beta.html
convert .dsv save file source: desmume, target: raw, size 512
output: .sav
open desmume, load pokemon platinum ROM
import backup memory .sav flash 4Mbit

happy playing