Why did nobody use Reaper Death Seal during the Fourth Great Ninja War? Why didn't the Reaper Death Seal appear during the Fourth Great Ninja War? If it was used on Madara or Obito, it would have ended the war easily, right?
Because there are only two known users of the Dead Demon Consuming Seal (also called as Sealing Jutsu: Reaper Death Seal): Hiruzen Sarutobi and Minato Namikaze, and the jutsu can be countered. 

While something like this could probably be done:


Use Summoning: Impure World Reincarnation to resurrect Minato and Sarutobi
They, then, use Dead Demon Consuming Seal on Madara
If successful and if the user desires it, release Minato and Sarutobi using Dead Demon Consuming Seal: Release so they can break free from the Shinigami and can go the afterlife


This is on the assumption that:


Reincarnated ninjas are not subject to limitations.
Madara cannot counter the Dead Demon Consuming Seal.
Only certain people can be broken free from Dead Demon Consuming Seal: Release (assuming that the user would want to release Minato and Sarutobi again).


However, we know that:


There are also limitations to reincarnated people as seen here, and while it was not shown in the manga, it may or may not be possible that reincarnated ninja cannot use Dead Demon Consuming Seal. This begs the question: Can a ninja who already offered up his/her soul to the Shinigami offer it again? There is no precedent to something like this in the manga so I am not sure we would know this any time soon, although personally, I believe the answer to this is a 'no' since Kishimoto did not let us see the characters attempting this.
Dead Demon Consuming Seal can be countered, as seen during Orochimaru and Sarutobi's fight. It's quite impossible that Madara would also not be able to do the same. This, to me, is probably one of the biggest reason why we didn't get to see it. A villain who's easily defeated by this do not make up for a good plot since this would have resulted to a much shorter and anticlimactic ending.
Cutting open the stomach of the Shinigami releases any souls inside of it. This would mean that Minato, Sarutobi and Madara would have to be sealed forever inside the Shinigami. You might argue that they would probably agree to this plan but that would have resulted to a less happy ending to the Naruto we know.