ffmpeg - Video - Resize - Convert to 4K (3840x2160)

ffmpeg -i input.mp4 -vf "scale=3840:2160" -c:v libx264 -crf 23 -c:a copy output.mp4

For enhanced quality

ffmpeg -i input.mp4 -vf "scale=3840:2160:flags=lanczos" -c:v libx264 -crf 23 -c:a copy output.mp4

NOTE: This uses the Lanczos scaling algorithm, which can provide better results for upscaling.