Slots 3 and 4 share bandwidth, is just 4 lanes of PCIe 2.0 (~1500MB/s) available for both slots.
A better approach is to move your GPU to slot 2, will lose access to slot 3, and install a switched adapter like the Syba/IOCrest on slot-1. This way you will have around 3000MB/s. If you need even more, look at HighPoint SSD710x cards.
You will need to upgrade your BootROM to 144.0.0.0.0, see the sticky thread about Mojave/BootROM upgrade.
With a good NVMe blade like 970 EVO Plus you have enough bandwidth to keep system and user/data files inside one blade, but maybe it’s better to have a separate scratch disk for your audio work, you will have less latency and more bandwidth.
Based on your idea (and reality=my bankaccount) I came up with the following scenario:
Since you mentioned the BootROM ,I understand that with this you can move the GPU from slot 1 and still the MP boots (which I thought impossible).
Then as a Low Budget version I could put:
Drive1- slot 1 Max. speed of the 970 EVO/Kryo
Gpu- slot 2
Drive 2- slot4 Speed limited to 1500 R/W 970 EVO/Kryo (scratch Disk/Audio)
MIght this work as well ?