While I don't have an answer that I like, I can share my technique for dealing with this problem.
I have the same setup. It's not Lightroom, it's that for what you are wanting to do, long exposures at high ISOs are not usable...except to calculate the proper exposure. So, get the exposure right at a high ISO, then make a longer exposure at a lower ISO and, poof, no ugly artifacts.
15 seconds at ISO 4000 = 960 seconds at 125 ISO, but you can recover 3 stops of the shadows at 64-125 ISO, so you might be avle to get away with 600 seconds. Of course, if it's getting lighter out due ot sunrise, it would be even shorter.
If you want, when the long exposure at low ISO is done, shoot some high ISO frames for blending purposes. IE, ripples on the lake etc. Best of both worlds. Here's an example...