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Given a polynomial algebra in some variables, say $\mathbb{Q}[x_1,x_2,x_3,x_4]$, I'd like to define certain subalgebras that are generated by elements, e.g. $x_4$, $x_3^2-x_2x_4$. So I'd like to define $\mathbb{Q}[x_4,x_3^2-x_2x_4]$. As this uses the variables of the bigger space, I assume it needs to be defined as a subalgebra. But I could not seem to find a way of doing so.

I'm actually just interested in the dimensions of the degree-graded spaces of the subalgebra if that makes it easier.