$4/kw makes sense electricity costs are insane i don't remember the exact but I'm pretty sure my cost is somewhere between 0.38¢ - 0.48¢/w
So power efficiency is hella sexy
Things i kept in mind are
1) no rgb
2) no dgpu
3) external das will likely use more power than internal drives and possibly have other issues
4) Wolfgang has a spreadsheet with psu efficiency and 20/40/60/80w so you can make sure you have a decently efficient psu at idle
5) try not to go overboard with case fans, just because there is space for it doesn't mean it's needed.