Options for 1 x 4K + 6 x HD on GTX970, GT740 and i5 6600K?

I've purchased an i5 6600K with an ASUS Z170-K (and an M.2 SSD which limits motherboard choice); I already have a GTX970 and a GT740. I have six monitors and I'm considering getting another 4K monitor (wasabi mango from Newegg). What kind of options do I have for monitor configurations?

Ideally I'd like to have at least the 4K monitor for the middle and 4 HD monitors for the sides. Can I run the 4K monitor and three other monitors off the GT740 and run three monitors off the GTX970 for gaming?
Can I use both the onboard video and the GPUs together?

This is mostly for software development but I do game some and would like to have as few monitors on the GTX970 as possible. I might have enough budget for another card like a 960 or 970 if I need it. I can also switch parts as I'm not opening anything until Christmas. I picked the Z170-K so I'd have 2 x PCIe16 + M.2.

mate you wont be able to run a 4k monitor and 6 1080p monitors on fucking 970, a 970 cant even play 1 4k

Actually a GTX 970 can handle 4K just fine, as long as you aren't trying to play AAA games (although I've seen a couple of cases where people are running GTAV on high at 4K with a 970) and @lulu58e2 said he doesn't play many games, it's going to be for programming. In terms of the GT740 I doubt it could drive the 4K monitor even at 30Hz, which it would have to do given it doesn't have HDMI 2.0 or DisplayPorts, you will probably be fine running 2-3 1080P monitors though. given the amount of monitors you want to use though it may be a good idea to get another 970.

depends if your happy with console performance

30 fps would be find on the 4K; I just want to see LOTS of code. If I can get the 740 and motherboard to run most of the monitors then I can leave just one HD monitor on the 970 and gaming should be okay. I play Ark and 7DTD and I don't mind 20-30fps on those. I'll see how far the 970 prices have dipped and maybe get another. Had I known I was getting a 4K monitor I might have looked harder for a 3 x PCIe 16 so I could use my 640 as well. Maybe I should exchange it.

Also, I'm currently running 3 HD monitors on the 740 and 3 HD monitors on the 970. They both work fine and I'm getting playable framerates in games on the 970.

@lulu58e2 You could probably run the 4K monitor off of the on-board graphics since you don't plan to play games using it and then run 2 x HD on the 970 and another 2 on the 740, or 3 x on the 970 and 1 on the 740.

@flamesilver Have you seen any documentation anywhere about running discrete and onboard graphics at the same time? If I can do that then I'll be laughing. I've seen dribs and drabs but no solid tutorials or reviews where people are running them together.

I just bought the Wasabi Mango 42 inch 4K monitor with HDMI 2.0 so we'll see how it all goes. Worst case I can save up and get another 970 or something.

@lulu58e2 I found this video: https://www.youtube.com/watch?v=N8nMShuSfgI

It's a couple of years old but might still be useful. Essentially the iGPU is the primary graphics driver and then that allows it and PCI-E cards to function at the same time. When you go to play a game you'll probably have to change some settings that way it actually runs using the 970 instead of the iGPU