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
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