OP, as Strelok said in posts #10 & #12, try hooking up with an ethernet cable just to see if you will get 4K comparable to the native Samsung TV app that way. You posted that your router is only 8 feet away from your

TV, so this should be easy to try. And as he shared in #10, this is NOT about making it a permanent connection- just testing to narrow in on the possibilities. Run speedtest this way too and see if you get something more comparable to what you are seeing on other devices (a whole lot better than 2.59 down)
Having swapped out

TV and still getting the exact same results, odds in 2 devices having the exact same poor wifi performance are very long. So that shifts some potential culprit to other hardware.
Since speedtest is coming in relatively poorly on

TV, the usual suspect of perhaps a poor

TV HDMI cable is less likely, but that's easy enough to swap out and test anyway.
So my guess (seconding waw74's in post #5)- as unlikely as it may sound- is going to be that the

TV's
location relative to the router is having some kind of wifi signal block/disruption. So another test is to temporarily move the

TV to a few other spots and run speedtest at each spot. I know that seems very unlikely given that PS4 is close to

TV and it is receiving well, but maybe there's something in that closet/wall/door that just happens to be disrupting the signal to

TV but not so much to PS4? Yes, that is unlikely but the idea is easy enough to test by moving your

TV around and seeing if it ALWAYS has poor wifi speeds no matter where you place it.
If it's always showing about the same wifi speed at
any location, I shift suspicions to the router. In this case, something in the router may not work well with

TV even though it works well with Samsung TV (app) & PS4, etc. Again, I suspect this is pretty unlikely, but all of these steps are process of elimination. Since you probably can't return the router, unhook your

TV and take it to a friend's house (where the friend is using a different router). Speedtest there and see what you get. If the speed ends up the same there, I shift my suspicions back to

TV and start thinking you've got 2 dud units in a row (unlikely, but it could happen). If so, swap it out again.
One more thing: are you sure your

TV is connecting to your own router? For example, if you live in a type of home with close neighbors (condo, townhouse, apartments), are you sure your

TV is not connecting to
their router? If you've named your router something unique, this is easy enough to check or you can do it by IP addresses. But the concept- as unlikely as it may seem- is that the Samsung TV, etc ARE indeed using your own wifi but your

TV is connecting to a neighbor's slower/weak wifi.
Lastly, a lot of these kinds of threads get resolved only by trying the suggestions of the people helping you. If you just reject suggestions (as you seemed to do for Strelok), nobody can do much more than just take wild guesses at possibilities. On the other hand, if you try suggestions and report back, process of elimination will lead to the specific link in your chain that is not functioning properly. If you want help, try what the help suggests and we can figure it out based on test results (narrowing in on the (probably) one thing that is the source of your problem).