Cylinder 4 misfire

There could be any number of reasons for a misfire. A code is a symptom, but the root cause should be determined through investigation first before just throwing parts at a code in the hope it will go away. It could have been the spark plug, or the coil, but it now seems clear that it wasn't. Bad carbon build up, a clogged injector, a dirty air filter or borderline fuel pressure pushing a marginal cylinder over the edge, or other stuff I don't even know to tell you about could be the cause. Hopefully the dealership will do a proper root cause investigation before the next round of parts go into it. Best of luck.

To add: A dealership has access to service documentation from the manufacturer that includes troubleshooting flow charts to drill down to a root cause. Of course the dealership mechanic may take shortcuts anyway (they shouldn't, but people are people), but at least they have the right tools at hand to drill deeper to figure out what's what. A third party mechanic may or may not have the right flowcharts and tool sets for a specific vehicle, and even if they do, they won't have the depth of experience with that particular vehicle that the dealership mechanic does. At this point, I'd say try a dealership. If there's multiple Hyundai dealerships in your area, then check the reviews in Google maps to figure out which one's service department people are most happy with. Good luck.

Just to add there may be an issue with the ignition lead too.
As Kevin says the next round of fixes could be expensive: like injectors. Mine were swapped 4 years ago, followed by plugs, which may have been fowled by the injectors failing. There were a couple of people noting injector swaps, so may have been a bad batch.
While you are on super unleaded, you will get some benefit of smoother drive but also cleaners.
Might be time to use a fuel system cleaner in your fuel... Maybe Redex... Especially if you are out of warrenty.
Time to smell the oil in the sump for fuel... And a increase in level. Unburnt fuel can seep by the pistons... Again a few noticed this.
Have you seen any camshaft position sensor errors... I've had mine swapped twice on my 2017 phev... But last pair changed 2-3 years ago 25k miles later still OK.

Good luck. Keep us upto date.

In the end I had cylinder 3 misfire, then multiple misfires. I had hardly used the ICE(petrol motor).

Thanks so much for all of the advice. I really do appreciate it! Hyundai dealership is not close by but sounds like I need to make the effort. It does seem to run fine and my mpg is around 55 most of the time. Might try the fuel system cleaner first just in case. and I got the impression that super unleaded might be better a better choice than regular unleaded?

I used super unleaded, only because my car normally runs on EV and ICE kicks in when it's cold for very few miles, say 10 miles max. I think it definitely helped and was costing me £1 a month extra.