Here's a save file that has cleared the game, with this you can either start a brand new game on legendary difficulty, use the old file as a new game plus or use all of the unlocked options on the title screen, just as a note this isn't a 100% complete file. 
I have posted saves like this for the other games as well, I also host these files on steam guides, which you can view here

Yakuza 0 gameclear/Amon Fight save file 
A Guide for Yakuza 0
By: Jario
A cleargame save file for messing around with or for just skipping straight to legendary, also useful for people that have played before and would like all the title screen options unlocked from the start.

Yakuza Kiwami Amon/Finale Legend save files 
A Guide for Yakuza Kiwami
By: Jario
A premium adventure save file for fighting Amon and a save for the finale on legend mode, just for fun

Yakuza Kiwami 2 Amon, Premium Adventure and Amon Bouncer Fights save files 
A Guide for Yakuza Kiwami 2
By: Jario
An Amon save file, premium adventure save file, one has a lot of stuff completed on premium adventure hard, one's premium adventure on legend and another save that is near the end of the bouncer missions.

Yakuza 3 gameclear/Amon Fight save file 
A Guide for Yakuza 3 Remastered
By: Jario
A cleargame save file for messing around with or for just skipping straight to legendary, also useful for people that have played before and would like all the title screen options unlocked from the start.

Yakuza 4 gameclear/Amon Fights save file 
A Guide for Yakuza 4 Remastered
By: Jario
A Yakuza 4 cleargame save file for messing around with or for just skipping straight to legendary, also useful for people that have played before and would like all the title screen options unlocked from the start.

Yakuza 5 100% Complete Save File/Amon Fights save file 
A Guide for Yakuza 5 Remastered
By: Jario
A Yakuza 5 100% save file for messing around with or for just skipping straight to legendary, also useful for people that have played before and would like all the title screen options unlocked from the start.

Yakuza 6 gameclear/Amon Fight save file 
A Guide for Yakuza 6: The Song of Life
By: Jario
A collection of save files for messing around with or for just skipping straight to legendary, also useful for people that have played before and would like to skip straight to legendary.

Judgment 100%, Amon Fight and Achievement Save FIles 
A Guide for Judgment
By: Jario
A collection of save files, including AN 100% CLEARED SAVE FILE, achievement save files and an Amon save file. These are all on Hard difficulty but can be used to create a legend mode save.

Lost Judgment Amon Fight and Achievement Save FIles 
A Guide for Lost Judgment
By: Jario
A collection of save files, including achievement save files and an Amon save file. These are all on legend difficulty but can be used to create a new game plus save of any difficulty.



Issues with selecting NG+ or EX-HARD
Seems that due to the way the save system works using these files doesn't unlock these options, so in alt save files is starts on all difficulties on ng+, use these instead of trying to start a save in game.




For the achievement saves:
Save 11, most completed, go to Akiyama's club and you'll get the Business Card Hunter achievement
Save 10 is one fight away from star of the coliseum achievement, do one tournament fight to unlock it
Save 9 is in the Dojo on the 2nd fighter, complete his arc to get the achievement



For the Amon save file, the first save lets you watch the first cutscene for the Amon substory, just go to New Serena, the 2nd save file is right after that cutscene and has everyone geared up and inventories packed full of staminan royales, for that save file you will just have to go to the underground parking lot and find a man waiting by a car, also make sure to go into your inventory during each fight just to make sure all equipment is actually equipped. The 4 amons are pretty easy, only the fourth one really poses much of a threat, using your roll forward heat move is a good way to drain his health. also in that save file, akiyama has guns, you might wanna replace those with more health drinks since he cannot use guns, found that out the hard way lol.

After downloading the folder, unzip it using 7zip or winrar, inside is a folder called yakuza 4 saves open the folder and copy the files, go to where steam is installed and click steam, userdata, [your steam ID], 1105500, remote and paste the files there. I recommend backing up your own saves, just incase. You might need to turn off steam cloud for Yakuza 4 for this to work, to do that, right click on Yakuza 4 in your library and go to properties and you will see a tickbox under steam cloud in general. Enjoy, kyoudais