Nothing wrong to get 6/8 core or even xeons, if You still need CPU power (doing some heavy workloads),..heck even Xeons up to 18Cores/36Threads. But..for Octane Render workstation/rig/node there's nothing bad to get quad core with Z97/Z87/Z77 or even older if..
..if the board has anything like PXL chip (PEX-8747) to help them handle lane splitting so You can can house 4x GPUs with 8lanes each.. & more PCIe slots to be able to plug three or four rahter than one or two GPUs (that is usual for motherboards based on these chips).. - Your CPU would have way less of lanes, but at least "highway" for data trafic is layed out on the motherboardboard so CPU can cumunicate to GPU without too much of bottlenecks if needed.
one of products to mention could be Z97 Gaming GT from Gigabyte - 4way caplable motherboards are nishe products, as games don't scale good enough with more han 3 cards, but it seems that every company has simmilar models like ASRock EXTREME9 - YOu can find some drom ASUS, etc - simply look through the sheets for 4way SLI support (that needs at least 8x).
Rendering performance is not influenced now even if You have Your GPU plugged into x1 instead of x16 lanes..but upload speed might be, especially if You work with heavy scenes.. However if You're investing into tool for work paying 50 $/EU more to have PLX onboard is well worth in perspective of whole system budget.
Then again, You can get X99 mobo (that would cost more), 6/8Core (that would cost more) & required DDR4 RAM (that cost mroe than DDR3) - but think wheher You need all that =) building a full PC is like tailoring the suit that fits Your shape rather than buying shiny things - unless You have too much money to blow & care only for show of..-think a head & it will save You some penies for additional GPU or better cooling that You would feel the effects in working with Octane.
Personally running Z77 + low end (3570K @4.5) OCed quad, & the only reason to switch for upcomming Z170 for me would be that & really want new NVMe SSDs in m.2 slot ..
hope that helps.