With 2TH it will take you approximately 1650 days on average to find a block at current difficulty.
Remember that this number is based on todays calculation. In prospect to the future you should at a min. of 10% - 15% to this number.
IF you bought some cheap S4 (2TH) at 300 USD, it could be a waste of your money, since it's not likely that you will ever earn something substantial towards ROI.
However, if your luck is to find 1 block before july 7 (the halving), your profit would be unmeasured. USD 8250 -/- your investment ;0