Is there a way to reverse migrate from gen 4 to gen 3 or restore your starter?

[lotus412 0]

Came in possession of a water damaged Emerald in which I migrated everything out of it as I did not have a link cable at the time. Is there a way to convert pk4 back to pk3?

[Poke J 223]

23 minutes ago, lotus412 said:

Came in possession of a water damaged Emerald in which I migrated everything out of it as I did not have a link cable at the time. Is there a way to convert pk4 back to pk3?

There is no legitimate way to convert pk4 to pk3, but if you can dump both save files you could use PKHeX to remake the Pokémon on the gen 3 save. There is also this tool that can convert pk4 to pk3, but you still need to be able to dump both saves. 

https://projectpokemon.org/home/files/file/2240-pk2pk-a-time-capsule-transfer-tool/

[theSLAYER 3366]

pretty sure PKHeX in HAX mode can convert down. It won't be legal, but it can be done.

[Sabresite 830]

To make it legal you need to know the location it was caught and the trash data. For events the trash data hasn't really been discussed publicly. For in-game it is dependent on gameplay.