4x GTX 1080 for Octane Render Station

How about this build (color theme: white).

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i7-6800K 3.4GHz 6-Core Processor ($428.99 @ SuperBiiz)
CPU Cooler: NZXT Kraken X61 106.1 CFM Liquid CPU Cooler ($114.99 @ B&H)
Motherboard: MSI X99A XPOWER GAMING TITANIUM EATX LGA2011-3 Motherboard ($363.98 @ Newegg)
Memory: G.Skill TridentZ Series 32GB (2 x 16GB) DDR4-3000 Memory ($159.99 @ Newegg)
Storage: Samsung 950 PRO 512GB M.2-2280 Solid State Drive ($315.62 @ B&H)
Storage: Hitachi Ultrastar 7K3000 3TB 3.5" 7200RPM Internal Hard Drive ($79.50 @ Amazon)
Video Card: MSI GeForce GTX 1080 8GB Founders Edition Video Card (2-Way SLI) ($647.82 @ Amazon)
Video Card: MSI GeForce GTX 1080 8GB Founders Edition Video Card (2-Way SLI) ($647.82 @ Amazon)
Case: Corsair 760T White V2 ATX Full Tower Case ($166.99 @ Best Buy)
Power Supply: SeaSonic X Series 1250W 80+ Gold Certified Fully-Modular ATX Power Supply ($174.99 @ SuperBiiz)
Optical Drive: LG WH14NS40 Blu-Ray/DVD/CD Writer ($48.88 @ OutletPC)
Operating System: Microsoft Windows 10 Pro OEM 64-bit ($109.98 @ My Choice Software)
Total: $3259.55 / $4555.19*
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-10-11 00:05 EDT-0400

* Note: since i wasn't able to add 4 GPUs at pcpp site, the final total would be 2 * 647.82 = 1295.64 + 3259.55 = 4555.19
It would be about $100 cheaper than your initial build.

Little description too.

CPU
Put in a LGA2011-3 socket CPU since for your work, you need extra cores and threads the i7-6800K offers.
comparison: http://cpu.userbenchmark.com/Compare/Intel-Core-i7-6700K-vs-Intel-Core-i7-6800K/3502vs3607

CPU cooler
Put in the best AIO water cooler. Only the custom made water loop is better than the Kraken.
specs: https://www.nzxt.com/products/kraken-x61

MoBo
LGA2011-3 socket CPU deserves a great MoBo to match it's performance. It comes with full 4-way SLI/Crossfire support with 8 RAM slots and a nice white theme.
specs: https://www.msi.com/Motherboard/X99A-XPOWER-GAMING-TITANIUM.html#hero-overview

RAM
Picked the same RAM as you did, 2x 16GB, 3000 Mhz. Leaving you 6 RAM slots free for future upgrades.

Storage
Switched your SSD out for a much better performing unit from Samsung.
comparison: http://ssd.userbenchmark.com/Compare/Intel-600p-Series-NVMe-PCIe-M2-512GB-vs-Samsung-950-NVMe-PCIe-M2-512GB/m168978vsm38554

Also switched out the WD 3TB HDD since it has high failure rate (not as high as Seagate HDDs but still quite high). So, put in a reliable 3TB HDD from HGST (Hitachi).

GPU
4x GTX 1080, Founders Edition from MSI to match the nice color theme this build has.

Case
Switched out your case for a better unit from Corsair. It's slightly larger, has front fan controller (2 speeds: low/high) and fits nicely with the white theme. But the main reason why i picked this case is because you can have a very easy access to your case's internals.
Here's 5min video review of the case: https://www.youtube.com/watch?v=s6UfXQHhR5k
specs: http://www.corsair.com/en/graphite-series-760t-arctic-white-full-tower-windowed-case

PSU
Put in a great 1250W unit from Seasonic.
specs: https://seasonic.com/product/x-1250/

Optical drive
BD/DVD/CD writer. Different model and a bit cheaper than the one you picked.

OS
Win10 pro 64-bit as you picked it.

I didn't include any case fans into the build since the case i picked comes with 3 pre-installed fans: 2x 140mm front and 1x 140mm back.

As far as thermal paste goes, any decent CPU cooler already comes with either it's own thermal paste or the thermal paste is pre-applied. But if you still think you need some extra thermal paste then don't go with Arctic Silver 5. It's one of the worst thermal pastes out there. Instead, i suggest you to get non-conductive Arctic Cooling MX-4.
info: https://www.arctic.ac/eu_en/mx-4.html

Here's also a bench between your build (base) and my suggestion (alternative).
For your needs, the "Work" criteria should be as high as possible.

Userbenchmark PC Build Comparison

Baseline Bench: Game 123%, Desk 94%, Work 75%
CPU: Intel Core i7-6700K
GPU: Nvidia GTX 1080
SSD: Intel 600p Series NVMe PCIe M.2 512GB
HDD: WD Blue 3TB (2015)
RAM: G.SKILL Trident Z DDR4 3000 C15 2x8GB

Alternative Bench: Game 139%, Desk 120%, Work 110%
CPU: Intel Core i7-6800K
GPU: Nvidia GTX 1080
SSD: Samsung 950 NVMe PCIe M.2 512GB
HDD: Hitachi UltraStar 7K4000 3TB
RAM: G.SKILL Trident Z DDR4 3000 C15 2x8GB