The total hashrate of the network keeps increasing. The more devices participate, the more calculating power there is.
The network adapts, by increasing the difficulty, so that blocks keep coming at 10 minute intervals on average.

Your 4 miners have a certain hashrate, which doesn't change. However, your proportion of the total hashrate gets lower and lower, since the network gets bigger and bigger. It's normal for your income to get lower over time, and it's the nature of bitcoin.