IMO, it's always best to render at your intended final resolution. You can scale up or down by, say, double or half size, but there will always be compromises when you do.
Most of the time, you can double the size of an image (4 x total pixels) and still get
decent results but, compared side-by-side with an image rendered at the intended resolution, you will notice a difference.
You can render at a higher resolution and scale down but why? You would increase the render time by a factor of four only to throw a lot of your work away when you scale down. Changing the number of samples or your de-noising settings to a lower setting will only degrade the quality of the image. Scaling down won't erase the resulting imperfections. (GIGO = "Garbage IN - Garbage OUT.")
My suggestion is to render at your intended final size then adjust your sampling and de-noising settings until you get an acceptable image with a render time you can tolerate. Make a test render at whatever settings you think are right then make a few more renders with different settings until you find something you like. I suggest using a "double and half" process.
Make your first test at, for example, 128 samples. Look at the image quality and consider your render time. Make a second test at 64 samples then compare the two images. If it doesn't compare with the first to your liking, try 96 samples. Do a couple of others at, for example 72 or 80 samples then pick the one you like best.
While you're doing this, look at the render times. Maybe the first, at 128 samples, looked good but it took too long to render. Maybe the second, at 64 samples, rendered quickly but looked crappy. Gradually home in on a number of samples... 128-64-96-72-80, etc... until you find find something that works. Look at the render times... 10 min. - 2 min. - 4 min., etc... until you get an acceptable time. When you finally hit your mark, lock those settings in and use them for your final work.
In my experience, many people have a tendency to make oversized images at way to high sample settings. They'll try to make images at 4K resolution with 1,000 samples then complain that it took ten hours to render one frame. Then, with all that time spent, they'll post it on the internet, thinking that they have a great picture but nobody really notices. The reason is that you have virtually no control over how your image will be viewed via the internet.
If the guy who views your picture has a high-rez-4K display then, great! What if the other guy is viewing on his iPhone? The latest iPhone has an approximate 2K-rez screen. (2796 x 1290) No matter how you slice it, that guy's iPhone is going to throw away 50% to 75% of the data from your picture. You will have taken four times longer to render the image and it will take longer for the viewer to download. It will waste bandwidth and storage space on servers.
Why would anybody with half a brain do that? Just to satisfy their ego? Just to prove that they have a great, big computer with a super-duper high-rez display? Nobody but nobody will really care in the end. They'll look at your picture for ten seconds then swipe left.
Do yourself a favor. Save yourself a lot of time and trouble. Show people that you know how to make interesting images that download quickly, that don't waste bandwidth and still look good.
Tailor your image size to the display you intend them to be viewed on. Most of the images that I make have a size of approximately 1536 x 1024. IMO that size produces an image with an aspect ratio and resolution that fits closely with most computer, television and smart phone. It probably won't fit perfectly on every display but it'll be close enough.
Are you making images to be displayed on smart phones? Television screens? Computer displays? Digital cinema? Maybe just general fooling around on the internet? Decide on your target audience. Make an educated guess at what kind of device your audience will be watching at. Make your images to match.