After Effects workflow!

Hi everyone! I'm pretty new to After Effects and I've been trying too Google/YouTube my question but haven't found a solution yet. So I'll give this a try.

The problem:

So, I'm trying to get my music video together. I have 15 3840x2160 video clips that I'm going to KeyLight and integrate various solids/masks and other effects such as lights cameras etc. The final render will be 1920x1080.

I have tried to use proxy but after just ONE clip of Keylight-effect and 5 second footage the preview is going really slow and sometimes even crashes. I use quarter quality for both comp and preview and a 5 frame skip.

I have AE installed on Z, and cache/workfiles on F: external HD (Not ssd). My C: (windows drive) is an SSD.

Should i change the installfolder or using cache etc differently for better result? (For easier workflow/less lag)

Dont have my exact Specs but its i7 2.6 ghz

16 GB memory

Geforce gtx 4gb

What's the best format/size for proxy when I wanna key out green screen clips (15 of them) and do I do the entire project in AE or do i first import them to Premiere, key them there and then replace them with AE-comps? So many ways to do it but what's the best way to do it so my computer won't get sad.

Hope anyone can help me!

Thank you!!