After the last attempt to get the Origin titan, in the battle of Shiganshina, why did Marleyans wait for 4 years to try again after their defeat?
The short answer is that the nation of Marley had suffered great losses in the battles leading up to, and including, the Battle of the Shiganshina District.
In the fourth story arc of Attack on Titan manga, Chapter 33,
Annie Leonheart, the Female Titan, is forced to crystallize herself prior to being captured by the Eldians on Paradis Island. In this way, she prevents herself from being consumed by the Eldians but she also can not help the Marleyans in their military operations either.
Then, in the Battle of Shiganshina District, we see
Levi Ackerman take on the Beast Titan, rendering both him and the Beast Titan incapable of further combat as both were considered to be at their limits.
Of course, elsewhere during the battle
Eren and his teammates take on both Reiner and Bertolt incapacitating both with new technology (thunder spears) and excellent military tactics.
This left the Beast Titan with two choices:
Save Reiner, the Armored Titan, or save Bertolt, the Colossus Titan. He chose to save Reiner and left Bertolt to the Eldians on Paradis Island. Armin, at risk of dying, was injected and chosen to consume Bertolt. And now, on behalf of the Eldians on Paradis Island, Armin is now in possession of the Colossus Titan.
So let's recap the events up until now:
The Marleyans were never able to recover the Founding Titan. They also lost the Attack Titan many years before Eren was born. Eren then became in possession of both the Attack and Founding Titan. Armin took possession of the Colossus Titan and the Eldians on Paradis Island are in possession of the Female Titan who, in crystallized form, is incapable of performing any further military operations of behalf of the Nation of Marley. That means that, of the 9 titans, four of the titan-shifting powers remain in the hands of the Eldians on Paradis Island. Not only was the military operation to recover the Founding Titan a total failure, but the Marleyans also managed to lose two of their titan-shifters in the process. To be fair, they actually lost THREE of their titan-shifters but Ymir willingly gave her titan-shifting form back to the nation of Marley so that brought their losses back to two.
Up until that point, the only reason the nation of Marley
wanted the Founding Titan was to take possession of the natural resources found on Paradis Island without suffering casualties as a result of the mindless titans roaming the land.
However, four years after the Battle of Shiganshina District,
Marley's battle against the Mid-East Allies proved that their military power was weakening; human technology rendered the power of the titan-shifters almost obsolete. Thus, they decided to continue operations against Paradis Island at great risk to themselves in order to bring all titan-shifters, including the Founding Titan, within their power. Of course, realizing that technology would soon make their titan-shifters obsolete, the Marleyans are only pursuing this plan to buy them more time; the titan-shifters will give them enough time to improve or advance their own military technology.
So the long answer is
the Marleyans had no intentions of returning to Shiganshina. They did attempt some naval operations but the ships they always sent never came back and so, they essentially gave up plans for Paradis Island. Then, four years later, the battle against the Mid-East Allies gave them greater motivation to go back and retrieve the remaining titan-shifters.