This mod is quite simple, It replaces the Merchant Outfit (the yellow one in the shop) with Haytham's Overcoat, a "rare" retexture that was only available to those who had a code which came from buying a figure when the game came out and it also replaces the Officer's Rapiers (14000 reales in the shop) with the Kenway Family Swords.

Effectively, It pissed me off that you couldn't get the cool trailer swords or a nice color on the outfit, so I seeked the help of SgtJoe who helped and made the data files required. You can find his page here with a bunch of other cool mods (Give 'Proper Pirate' a look, it's pretty slick).

This (Rapiers)

is visually replaced by

This (Kenway Unique Swords)


AND

This (Merchant Outfit)

is visually replaced by

This (Haythams Overcoat)


                        
This mod is probably best to work alone since the files I have provided just fully replace a few of the forge ones running the game, while I'm sure you could get it to work with something else, it probably is best not to. If you are savvy enough to use the Delutto Tools You are free to recompile the data files I provided into any forge file you want, meaning you can combine it with basically anything that doesn't replace the two items already.  

Because I appreciate modders for providing a dummy strategy, I will do the same in the form of two files you can just drag and drop to do the model swaps.

Alongside this dummy strategy I will also Include the .data files that you can use if you don't feel like downloading a 1.9gb file most of which you don't already have. Check the files page to see which to download.

The swords and outfit can be done separately if you don't care for one or the other.

Dummy Strategy:
To install the mod, download the two files labeled dummy strategy, and drag and drop them into your AC4 install location (C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed IV Black Flag) This is what mine looks like, so aim for something similar. For you plebs on the Uplay launcher I have no idea, it probably works identically the file path is just gonna be different. The only downside to this strategy is the download size.

Delutto Strategy:
This one is a bit more difficult, requiring you to use the Delutto forge tool that I linked above to deconstruct the right files, slip the mod files in, and recompile it. For people who know how to use these I will include the files you need to put them into to prevent you needing to download 1.9 GB.

I will be reasonably responsive on this page and on my steam account if you need to ask questions.