You can definitely mine Eth with Nvidia... it's not their specialty, but at current exchange rates it doesn't really matter if they're not quite as efficient.

If by the Zotac Mini you mean this one --- https://www.newegg.com/Product/Product.aspx?Item=9SIA85V5RY5645&cm_re=zotac_1070_mini-_-14-500-408-_-Product --- They do quite well, in fact one of the 3 GTX 1070s I mentioned getting 25.5-26 MH/s mining Eth at stock settings is that exact GPU. The only downside to it is that they are much shorter, so we had to improvise to get it to fit on our open air mining frame.

There are allot of coins that can be mined aside from the big boys, Ethereum and ZCash, so if you decide that you like the whole mining scene there are other options that are very viable right now. 

We have some GTX 1070s here that seem to all test at 25.5-26 MH/S on Ethereum using the Nicehash benchmark. If you raise the memory clock on them they should do a bit better then that. They are certainly viable for Eth mining with the current exchange rates. If you buy Nvidia make sure to check out ZCash and all the other Equihash based coins.

Hi,
1: www.whattomine.com (fill in your 4 gpus, just select 4x rx470 since the site doesn't have rx5xx series cards yet, your power cost per kwh and hit calculate. Chech the results.
2: don't mod your cards if you don't feel "safe". Just run them at stock bios and play a little with memory clock (for etherium)
3: you can use poloniex, if you plan selling your coins. For long term storage get the local wallets for each coin.

That rig you describe would be profitable with ETH continuing to go through the roof. Your ROI in days then would be fully dependent on what you can get your GPUs for. Unfortauntely, right now, there is a world-wide shortage of 580/570s (apparently, AMD & OEMs did not predict the extra demand brought on by altcoin explosion) and 480/470s were discontinued a few months ago upon the release of the 5xx series.  I was lucky enough to get 8 MSI 470's right around that time-frame for a really good price, but when I tried again only a week later to get more, they were either unavailable or 3rd party sellers were marking up 2-3x which I will not pay.  I would say that if you could find any 4xx/5xx for under $300, pull the trigger. Even at that inflated price, you will still ROI reasonably as long as ETH stays at a high $$.

Word of caution, watch out about mixing 4xx & 5xx. I know there have been some major driver issues trying to do this and not sure if they are resolved yet. Here is my recommendation on order to buy in if price is about the same: 470/570/480/580... maybe flip the 570 & 480 but 470 clearly the best all-around for ETH and 580 the worst as it uses way more power with no greater hash-rate vs 480.

Good luck finding these, i pretty much gave up in the US for now as all remaining 470s in the supply chain were bought up in several big shopping sprees where miners went out and bought multiple cart-loads of those suckers! Theres a thread here somewhere with pics... wish i would have had the foresight (and extra $) to do that but its all good.