1) f(x)= x4+5x3-x2-5x
Statements 1, 3, and 4 are correct. 2 is not because f(5) does not equal 0.
The Rational Root Theorem is:\
Let f(x) be a polynomial with integral coefficients. The only possible rational zeros of f(x) are:
pq
where p is a divisor of the constant term and q is a divisor of the leading coefficient.
So, fators of the constant on top and factors of the LC on bottom.
±1,±2,±4±1,±2
Now, pair them together.
Divide all of the top factors by 1 first, then 2.
±1,±2,±4,±12,±1,±2
Repeating factors can be dropped so the possible factors are ±1,±2,±4,±12 or (x+1),(x−1),(x+2),(x−2),(x+4),(x−4),(2x+1),(2x−1).
So your possible factors are correct.
To see which ones are factors, I woud graph it.
This graph shows that (−2,0),(−12,0),(2,0) are the zeros. (Here is the graph: https://www.desmos.com/calculator/fx9umcmzu0)
As factors those are (x+2), (2x+1), and (x-2).