4k Photos or just video? I've seen in the updates that 4k 60fps has been added but I cannot find a setting to take 4k photos. The pictures don't look as good as the videos but I don't know if I am doing something wrong.
The "60fps" should indicate that it's only for video, which is consisted of many (picture) frames per second (fps).

4K video itself means "roughly 4000-pixel wide". This term came after the term 720p and 1080p for video, which both indicate "720-pixel wide" and "1080-pixel wide" video (the "p" is for "progressive" though, not for "pixel", but that's outside of the scope).

Photos, on the other hand, are usually calculated in megapixels (MP), which means "the total number of pixels in million". Considering a Camera app usually take a photos in 4:3 or 16:9 aspect ratio (width:height), for 4K-pixel wide, they are roughly 4000x3000 pixels (or 11 MP for 4:3) or 4000x2250 pixels (or 8.5 MP for 16:9).

According to GSM Arena, Xiaomi Poco F1 has a main camera up to 12 MP and selfie up to 20 MP, so there shouldn't be any problem to take "4K photos". There should be a setting on the stock Camera app to set the MP. Otherwise, try 3rd-party Camera apps that are able to.