I would definitely try to look at both cars. If the cost is the same, personally, I would go with the audi. I've had German cars for years and they really are better quality. I recently downsized (monetary wise) from a nearly new merc c43 to a4 avant as I'd only driven 2k miles in a year and only insure for 6k miles normally. I'd rather see the money back in my savings than sat on my drive. I then changed again to the cupra estate as I missed the fun side (it is nippy), I also missed the 4drive and also didn't want future dpf issues with the a4 diesel and my low miles.
The a4 was a 2020 facelift and the cupra st 300 is a 2020 69 reg but I understand the Seat haven't been updated for quite a while which to me is obvious.
A few things I've noticed already, some I knew before I bought it.
Cupra carpets and mats feel very thin compared to others
Buttons and switches etc don't look or feel as good quality as the others
The materials/ finish on the dash looks cheap
Satnav. I've only used it once but I was looking for house number 217 in an area I'm unfamiliar with and satnav said it only went up to number 97.
Seats aren't as comfy. They feel smaller and again, not as good quality
The infotainment system isn't very good. The graphics seem cheaper and it seems quite slow
I'm used to having remote controlled boot door. It's like going backwards using the manual boot handle to open and close it the old fashioned way. No motorised parcel shelf either which isn't a necessity but noticeable
The bonnet doesn't have gas struts so it's like going back in time manually propping it up with the rod or whatever it's called
Just sitting in it feels inferior to the other cars I've had
Even the cupra key fob is cheap
Good things about the Cupra
It's a quick estate under the £40k tax bracket
Mine isn't the R so cheaper than past cars so I've put money back into savings by changing
It doesn't attract attention which suits me as it doesn't seem to attract boy racers and I don't worry about parking it up like I did my mercs
If I'd have had the choice of quick Seat or quick Audi at the same price, I'd have gone Audi
It sounds like I dislike the cupra, I don't but it's very different quality wise to the audi and you saying the cupra and s4 you are viewing are the same price.
Perhaps the R is a big step up interior wise from the model I have but if it isn't, I'd go audi