Even if you have free electricity ROI is hard to reach if you spend money for miners
In 4 years with free electricity, one still may not make their money back? I'm personally not a miner, and I understand it's too late in the game for the general population to get involved... but that sounds ridiculous. With free electricity, OP still shouldn't expect to turn a profit?
Some are pretty anti-mining. Best way to calculate ROI is something like this -
https://bitcoinwisdom.com/bitcoin/calculatorOP add up all costs including if you have VAT, etc. That will help you see how long a miner would take to pay off. Also when you say 4 years are you talking about some kinda student housing where you just have one room?