S4's are getting dated at this point as far as efficiency. How much free electricity do you have and is sound a issue? Those two things are kinda important.
I personally would go S5 if you can find a deal, or wait till having and I think you will get some bargin S7's. So honestly waiting till having might be a good idea to snag some S7's unless you find a deal on a older miner.
Sound & Heat are no issue...and "how much" free electricity... I dont know honestly but these would be placed directly into an already fully functional server room with an AC Unit and proper ventilation (it stays ~65 degrees in their). Not a HUGE room, id honestly even call it "walk-in-closet" sized. I dont want to do anything crazy (yet) and honestly was looking at the S5 but then I realized I had to buy power supplies if i got them and didnt really want this to start off taking up some space and figured the S4s in a clean AIO unit would work better for me (right now). Plus the deal i got on 2 of them was good so i just went for it.
With some good GPUs you can probably make some decent profit mining altcoins, especially if you have free electricity.
I was also thinking about this, I have a 290X sitting in a box at home and was thinking about Etherium or some other altcoin but would have no idea where to start with those, atleast with bitcoins ive done before and have been following for years now. (kinda)
Since you're able to avoid worrying about the electricity, find some used miners (or new ones if you really care), get a nice setup with a fair amount of miners, and start generating heat Bitcoin.
I wouldn't necessarily recommend putting in too much money early on, but as long as you think you can sell the miners for something similar to what you bought them for later on, you'll be making a fair amount without having to worry about losing much of your value at any point.
Yes this is something i thought about a lot. I definitely want to start off small and just build from there. I am not too worried about re-sell value of my current rigs but if i go with something more expensive I would definitely have that weigh more as a consideration. Current plan is to go the cheap used route to get generating, and as i feel more comfortable go on from there.