I have been using premiere cs4 since its release till now. However it is becoming very sluggish when editing 4K video so it's time to move to a more modern platform. I'm looking for something I can get for under $500 one time fee.

I usually shoot multi cam video interview. I also have audio recorded separately on an audio recorder.

I need the editor to
- split stereo into two mono tracks because I mic different people with left and right channel
- play audio clip slightly faster or slower (i.e. stretch) to sync with camcorder audio since the two machines have slightly different sampling rate
- export to 720p or 1080p for youtube
- have audio compression effect to limit audio dynamic range
- basic video and audio transitions
- basic color correcting between different camcorders
- very stable and doesn't crash often

I think these are very basic requirements and all NLE should be able to handle them, right?
So what's a good NLE to invest in (both money and learning)? I'm currently using windows 8.1 and eventually will move to win 10. I have an i7 4790 processor, 32G RAM, and a lowly nvidia GeForce GTX650 video card. But I will consider buying a faster video card if necessary.

Vegas pro? Lightworks? Davinci resolve? Suggestions on which one to pick?

Well the only way you're going to know how they work for you is by downloading a demo and trying it out to see which one works for your work flow.

I know I am using a basic editng software but it works with my work flow and I have done multi camera editing with it Corel Videostudio Ultimate x10. It can also render video to dvd and a load of other formats as well.

I also use vages 15 pro for other editing but I mainly use Videostudio. I was given Vages 15 Pro from a friend who bough it and though he would use it but he never installed and has given up the video work. I like vages for the overlays as in the news stile over lays for names of people or thing like that.

All I can say download a tril/demo and try them out to see what you like.

both of above are once of payment

demo is the way to go, but include Edius in your list - it's a great editing platform for what you want.

If you have stuck with not so long, why are your forcing a complete re-learn? I vowed I would never be a subscription payer, but frankly, the benefits far outweighs the annoyance. I still upgrade one piece of my software but when you look at the upgrade costs over ten years and the subscription - they're really very close, and with Adobe, you get the others thrown in - so I've got heaps of adobe apps, and use about 6. Premiere CC is so far ahead of the old CS4 that it's worth it. My accountant tells me subscription vs purchase really makes no difference. It's a cost of doing business. Adobe gave me CS6 to keep - it's on the computers, but why would I want too use it?

I hear good things about the free editor from Blackmagic Design.

I would certainly try resolve, their free version is completely usable and can help to decide if it's worth to upgrade to the studioversion, differences can be found here: https://documents.blackmagicdesign.com/DaVinciResolve/20180404-10399d/DaVinci_Resolve_15_Feature_Comparison.pdf
I am a edius user and have been trying resolve and was very impressed at how good it was, especially when you generate proxies which makes a huge difference when working with demanding codecs.

I've just made the switch from Premiere CS6 to Resolve 16, and wish I had done so long ago! The learning curve was nowhere as steep as I anticipated, and I am an (very) old dog!

As Noah suggests the free version is usable, but I ended up paying for Studio to get the extra effects.

I am sticking to Audition , as CS6 does all that I want, and I am very familiar with it, having used its predecessor Cool Edit Pro.

The fact that Fusion is now thrown in to the deal is excellent.