y=4x+4 could also be written as f(x)=4x+4 since y is a function. You can see this in graphs as y is represented or labeled as f(x) when graphing. This could easily be understood and solved especially if you have the value of x in equations. Let us take the above-mentioned equation as an example. x=1 therefore f(3)=4(3)+4 is equals to f(x) or y= 5.333
Hope this helps. Have a nice day!