Give an example of a four-dimensional subspace of $P_4$ which contains the polynomials $3 + 2t^2 - 6t^4$ and $1 - 2t + 3t^3$.
(1) Check that both your polynomials (vectors) are linearly independent
(2) Take your vectors' span: now they're contained in a 2-dimensional subspace of $\,P_4\,$
(3) If you really need 4th dimensional subspace, just add two polynomials (vectors) to your given two that are lin. independent from them...