In RGG4, for the first time in the series protagonists other than Kazuma Kiryu appeared. What should be noted is that Akiyama and Tanimura, who are not yakuza, have jobs. They are an endeavour to show professions within the game.
Shun Akiyama's occupation is being a money-lender. Due to his statement during the game that he doesn't care whether or not the money is returned to him, you can see that he is in possession of a huge sum of money. He seems to have earned more through asset management than loans. Masayoshi Tanimura is a detective - in other words, a government employee. If he avoids pay cuts or suspensions through illegal activities such as damage to property or accepting bribes, he should be able to make a decent amount of money. Also, you may assume that Taiga Saejima is out of work due to being on death row, but since he is moved to an Okinawan prison rather than a jail like before he seems to be treated differently under current laws. From his proficiency at making wood carvings as displayed in his revelation scenes, you can guess that his long years of diligent work in prison have paid off.
This brought to attention the fact that, unlike the other protagonists Kiryu, who has been the main character throughout the whole series, does not work at all. It is completely unknown how much he makes from running Sunshine Orphanage in Okinawa.
The representative source of income for yakuza is debt collection, managing underground casinos, serving as bodyguards for adult businesses and so forth. Sometimes they also engage in the trading of drugs, pimping, and organised fraud. But it is hard to think of these serving as a source of income for Kiryu once he becomes a normal citizen. Perhaps there is someone who aids Kiryu with running the orphanage. It is also possible to consider the idea that a former exceptionally intelligent yakuza who once took care of Kiryu is providing him with financial assistance... According to a Sega spokesman, "Kiryu's source of income is a secret." Unfortunately, Kiryu and Haruka's living expenses remain a mystery.
Travelling all over Kamurocho, starting in the afternoon you can play pachinko or spend your time alone at karaoke. You can beat up thugs around town and use the money they pay for inconveniencing you to eat gyudon. He behaves in a way unexpected of a protagonist. Perhaps a day will soon come when Haruka's request is for him to get a job?