iMovie does indeed support 4k, and can even output ProRes, assuming you have the latest version on a late-model mac with hardware that can actually handle it.
I use FCPX most of the time... iMovie on occasion on my secondary macbook when I'm travelling and want to do a quick edit and post.
As mentioned earlier, iMovie is "free" and included in every mac, FCPX is $300. For Windows machines I used to use Adobe Premiere in the past but I prefer the UI of FCP more, and I'm just happier working on an OS that is not windows in general. Adobe Premiere is not a bad option though if you don't have a mac. Obviously not free though.