Premier Pro CC question

[hkdman]

Hello - still learning how to use Premier Pro CC. Question - I typically film in 4k. However, I post my videos to Youtube (which only does 1920/1080p). When I render the video - should I convert it over to 1080p since that is the highest that Youtube can display or should I render at the higher 4k? Obviously 4k takes quite a bit longer which is why I am asking. Another question - if youtube can only to 1080p - then should I just shoot in that video quality? What is the purpose of 4k...is it just so that I have the video clips for when/if youtube or other sharing programs upgrade to 4k I'll be ready to post higher resolution videos?