No, not the same. The FSX version is native-FSX. The FS2004 is native FS2004, but there is forward compatibility FS2004 terrain to FSX. But there is not backward-compatibility FSX to FS2004.
One difference is that FS2004 terrain can only have one resolution per BGL file. FSX terrain can have all resolutions contained in the same BGL file.
FS2004 version is 19m horizontal resolution. FSX version is 10m horizontal resolution.
Hope this helps