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 a guide like this for all the other Yakuza, Like A Dragon and Judgment games. Links to all of them can be found here and on my profile. The links can be found here and on my steam page:
https://steamproxy.net/sharedfiles/filedetails/?id=2689025548
https://steamproxy.net/sharedfiles/filedetails/?id=2585769811
https://steamproxy.net/sharedfiles/filedetails/?id=2586740791
https://steamproxy.net/sharedfiles/filedetails/?id=2379715518
https://steamproxy.net/sharedfiles/filedetails/?id=2389968624
https://steamproxy.net/sharedfiles/filedetails/?id=2408950501
https://steamproxy.net/sharedfiles/filedetails/?id=2440359743
https://steamproxy.net/sharedfiles/filedetails/?id=2447367563
https://steamproxy.net/sharedfiles/filedetails/?id=2864943075
https://steamproxy.net/sharedfiles/filedetails/?id=2872921349

Seems that due to the way the save system works using these files doesn't unlock these options, use this guide which is a very end game save and finish it and it will unlock both options for you:
https://steamproxy.net/sharedfiles/filedetails/?id=2484226156

Clear Game Save:
https://drive.google.com/file/d/1528uIOU1jS5Gn9MbeN0MePimATzyLjra/view?usp=sharing

Amon Fight Save:
https://drive.google.com/file/d/1pRVofzw0uvt5FIrWaaAz23wfX9vBCeMY/view?usp=sharing

Alternate save files including new game plus file starts on every difficulty in a seperate folder incase the other saves don't let you start new game plus:

https://drive.google.com/file/d/1NMXcdTTcCEnEzkPlZjJJfhLC6YE1orJc/view?usp=sharing
For this one either use the saves in the ng+fix folder or delete that folder before moving to your save location.

Achievement Saves:
https://drive.google.com/file/d/1Pli8CWyKo0dVvJJAyNnBZMlZTZFbbopz/view?usp=sharing

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

All Saves:
https://www.nexusmods.com/yakuza4remastered/mods/54


Please rate and favourite if these guides have helped you! Awards are nice too but not necessary.

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 royale's, 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