Hello, thank you.
I took the constraint into account by substituting y=ax^4 and \dot{y}=4ax^3 into the Lagrangian: L = T-V = \frac{1}{2}m(\dot{x}^2 + \dot{y}^2) - mgy
To yield, without any small oscillation approximation: L = \frac{1}{2}m\dot{x}^2 (1+16 a^2 x^6) -mgax^4
However, when you try...