Convert FULL HD video to 4k

If you are using Premiere Pro, it can indeed upscale Full HD videos to 4K resolution. However, this process does not increase the actual detail or quality of the video. If you want to increase the detail of the video while upscaling to 4K video, you can try a dedicated AI video enhancement tool, such as AVCLabs Video Enhancer AI.

For years I waited for super-resolution software to bring low resolution film and video to HD and 4k.  I was most optimistic about the technology that added detail to the central frame interpreted from a few frames ahead and behind.  However, there is a new technology I have been experimenting with in 2022 developed by Topaz Labs.  It uses artificial intelligence "guessing" derived from a huge and growing online reference library to add detail that was not in the frame.  At low settings, it sharpens lines and curves much like AE Detail-preserving Upscale; at medium settings it begins to fill in features with details that were not in the original material, such as hair strands or distant faces; and at high settings it can be jaw-droppingly realistic or dangerously comical, depending on the material, so you have to be careful. I am presently renewing a 90-minute DV documentary I shot in 2000 by upscaling from SD 4:3 to 3840 x 3072, which is the aspect ratio of SD, riding a UHD 16:9 3840 x 2160 crop out, then down-scaling to HD 1920 x 1080.  For me, this is the new Holy Grail.

If you can put the footage into After Effects, then you can add the "Detail-preserving Upscale" effect to it, and that could produce a somewhat better upscale. Though, as Jeff said upscaling typically reduces the clarity.

The most basic way could be to simply choose an HD clip as a Source in Media Encoder, and Export as 4K. The same could be done in Premiere, exporting HD sequence as 4K.

However, converting HD to 4K doesn't make it look any better - you can't add resolution (detail) that did not exist to start with. Meaning, you might send the HD source to a 4K display, and then send the 4K converted video to same 4K display for comparison, and the "4K" source isn't going to look any sharper, so nothing gained.

Perhaps if you share the intended purpose/workflow behind your question, we can provide different solutions.

A similar question I see a lot is "convert SD to HD". Same result - one can play a nice quality DVD in a set-top DVD player (with HD-upscaling via HDMI cable) to an HD display and get a great-looking image. The results would likely not be any better if the DVD was upconverted to Blu-ray. The source is the source.

Thanks

Jeff