Since I do not know if by x2 you meant \(2x\) or \(x^2\), I'll solve for both.
Okay. here is the solution for when it is \(2x\):
If the length is 5ft, then \(x = 5\).
Using this to solve for \(5x^3 + 4 - 2x\), we get \(625 + 4 - 10 = 619\).
The perimeter is \(5 + 5 + 619 + 619 = \boxed{1248}\) ft.
Now, here is the solution for when it is \(x^2\).
If the length is 5ft, then \(x = 5\).
Using this to solve for \(5x^3 + 4 - x^2\), we get \(625 + 4 - 25 = 604\).
The perimeter is \(5 + 5 + 604 + 604 = \boxed{1218}\) ft.