If you just want to wire up a light bar then get the 4 way and your golden. But if you plan on towing anything I would get the 7 way - this way its future proof... you can connect to whatever trailer you encounter.
Trying to convert the 4 pin to 7 pin just to save $110 ($150 factory part vs $30 Curt adapter + $10 in misc connectors / wires / heat shrink) seems penny wise but pound foolish IMHO, especially once you factor in lying under the vehicle figuring this all out and connecting things up. Its certainly possible to accomplish this, so if you enjoy such projects go for it.
Once again, I have a 4-way already and a light bar. There is no way I'm buying a Hyundai OEM over priced 7-way and having to spice into it to connect the 4-way light bar plug at the rear. It is 100% easiler and cheaper to simply plug in a Curt 4-way to "Dual-Function" 4-way/7-way to your existing 4-way for this application (and I would argue for only a 7-way too, if you already have the 4-way). The 4 additional wires are only: reverse lights, electric trailer brakes, positive hot wire and ground.
If Hyundai had a plug and play harness
WITH plug and play controller (which they do not offer), then that would be great for those that don't already have a 4-way in the back and don't plan to run a light bar. You wouldn't/shouoldn't need a professional to plug the wires in.
To each their own application. This simple wiring is not rocket science. All owners need is correctly labeled plug knowledge. None of this should be presenting anyone problems once owners know what connections are what.
If someone doesn't understand how to wire those added 4 wires to make a 4-way into a 7-way, then they likely won't know how to wire the 4 wires at the front once they buy the Hyundai 7-way and an aftermarket controller. That has to be wired buy the owner no matter what rear harness you use.
I highly recommend those that get confused regarding proper wiring for their trailer brakes hire it out to a professional and get a warranty on the work. Even the professional can't do the job until he knows what all the Hyundai SC wires are for.
This stuff is a cake walk for the trailer professionals. Don't think once you buy the expensive Hyundai 7-way harness for the back and plug it in that you are done and ready to tow with trailer brakes.
I'm tapping out on this electric trailer brake stuff. Best of luck folks.