How to compress and convert 4K to 1080p easily and fast on Windows (10) PC or Mac for easier playback with the best 4K to 1080p converter? Thanks

i can suggest the use of "avconv"

avconv -i video4k.mp4 -s hd1080 -c:v libx264 -c:a copy video1080.mp4

or directly ffmpeg (a little bit tricky the command..)

ffmpeg -i video4k.mp4 -vf scale=1920:1080 -c:v libx264 -crf 20 -preset slow video1080.mp4

(you can find the ffmpeg on BIN folder of kdenlive installation directory)