I bought hdfury x4 to manage iptv box signal to my 4k LG tv few years ago. When I connected everything in place, I could tell immediately if there is a noticeable difference with or without. However, after a time the picture quality was consistently very good, crisp and it felt like colours got deeper. The picture looked much better comparing to similar setups I had or came across , which did not have X4 installed. Now I have upgraded my old iptv box with the new one, capable of 4k. However given the content is still 1080i or 1080p at best, I still feel the old box + X4 outperformed the new box without X4.The question was about a solution for 1080i material, Linker only support 1080p for upscaling to UHD, so my suggestion was to use HDF4S to convert 1080i to 1080p and then use Linker for 1080p to UHD, when bought together, we can offer the combo HDF4+Linker for 399$. Hope it's more clear, for the quality comparison, please buy and try, if you don't like it, send it back within 30 days as we cannot really answer that quality question for you, only real tests can tell you if it's better than your TV work.
It's simple, 720p or 1080i comes in HDF4S and it will output it at 1080p (this is one time setup only, once setup, anything you throw in, will go out at 1080p), which then comes in Linker which will output it to UHD up to 600MHz.
So basically SOURCE > HDF4 > LINKER > TV.
It's hard to answer on quality, it's really something which is unique to everyone, you might prefer this or that for various reasons, for ex, not everyone can tell difference between 24 and 60 fps but some can tell diff between 60 and 120.
Our suggestion for you is simply to try and make your own opinion as you can send back for full refund within 30 days (no restocking fee)