If y=4-1 is the equation of a straight line graph, what is the gradient? in advance.

I can see two different possibilities for what your question
really means.  Here are my answers for both:

Possibility #1:   Y = 4 - 1 ,  just as it appears.

Simplify the right side.  Using a scientific calculator, or a
multi-core computer running the latest advanced high-speed
forensic analytic number-crunching software, determine that 
4 - 1 = 3 .     Substitute for the right side of the equation:

                         Y = 3 .

In this form, we see that 'Y' is defined as a constant ... It's always '3'.
Since the value of 'y' doesn't depend on anything, 'y' never changes,
regardless of how 'x' or 'Q' or the weather or the stock market may change.

The gradient of a function is its maximum rate of change.

The gradient of 'y' is zero.



Possibility #2:  You may have intended to write   Y = 4x - 1 .

This is the equation of a line in Cartesian coordinates whose slope
is '4' and which intersects the y-axis at the point  (0, -1).

The slope of the line is the gradient of the function, i.e.  4 .