This is very simple and it's the EXACT same as every other Assassins Creed title, without trying to insult, but I'm amazed nobody has figured it out yet, I came here looking for a mod tool to avoid having to hex edit myself (call me lazy), but all I could find was a thread with someone trying to figure out how to hex edit past the 65k mark and some people still even struggling to get that far, well, I have the answers to your problems.
Part One - Getting to 65k
1) Get between 1,000 and 9,000 "Reals" on Assassins Creed IV.
Note: Make sure to write down how many "reals" you have EXACTLY before exiting the same.
2) Make the game save, either go to a sync point, find a chest, or something similar to trip the "auto save" feature.
3) Transfer your ASSASSINSCREED4SAVEGAME1A to your USB stick.
4) Open your USB stick with Horizon or some other device explorer (horizon is my personal favorite, because even its free features are still extremely functional).
5) Extract your ASSASSINSCREED4SAVEGAME1A to your desktop.
6) Make a backup. (copy the file and add .bat to the end of the file name).
7) Open your ASSASSINSCREED4SAVEGAME1A in a hex editor. (I used HxD).
8) Go to Register to see the link. It is a base converter that I personally used for modding this game, it's simple and easy to use.
9) Type in the amount of money you had into the "Decimal" part of the converter.
Example: 6,000 reals would be 17 70 in HEX.
10) Go to your hex editor, press CTRL+F and search for "hex values". This part is important, READ CAREFULLY.
You're going to want to invert your hex value, so for example if your hex value is 17 70 when you invert it, it would be 70 17.
Search for your hex, and it'll be highlighted, change both sets of numbers to "FF FF" this gets you to 65k.
Part 2 - Getting Past 65k.
1) Load your save back up and play the game until you get about 68,000 "Reals" then follow the same steps you did above for exporting your ASSASSINSCREED4SAVEGAME1A to your desktop.
2) Make sure you've taken a note of exactly how many "reals" you have before exiting the game, then go to the base converter again (link above, in previous steps) and input your value into the "Decimal" field.
Example: 68,000 REALS is 01 09 A0.
3) Open your save in your hex editor, and do another search for hex values. READ CAREFULLY.
With this specific edit, you'll have a hex that has two values in a set of 3, such as: 01 09 A0 when you search for this value, you want to take the first two values, "01", delete it from the start and add it to the end, so your hex would now be: 09 A0 01, the next step is to take the last two values (before you changed it) delete them, and put them at the start, so your hex should now be: A0 09 01
In simpler terms, you swap the first two values and the last two values around with each other.
4) Search for your hex, and it'll be highlighted, change all three sets to "FF FF FF" this will take you past the 65k mark and give you more than enough "Reals" to be "rich" throughout the entire game.
I'll be releasing tutorials on hex editing your resources, wood/metal/sugar/rum, etc. I'm guessing it'll be basically the same but I haven't tried it out yet.
If you want to rip this tutorial and post it on other forums to try and gain "likes/thanks" or just "respect" from other members, feel free.. I don't really care about anyone stealing common sense.
UPDATE - Getting Past 65k (part 2)
I have found that if you mod your save to the 65k mark, and then play the game properly, find some treasure chests, treasure maps, assassination contracts, just anything to get you money, get over 75,000 in-game, and then follow the steps from the second part of this tutorial.
IMPORTANT: Delete your Assassins Creed 1B savegame file when trying to get over 65,000. (at least, that was something I did and it worked).
If you aren't capable of hex editing, or keep experiencing problems, refer to this thread: Register to see the link