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Because of the order you've defined the objects, L(t=t) will be an element of R2. Slightly more robustly, in your example you can do any of the following.

sage: L(t=R2.gen(4)).parent()
Multivariate Polynomial Ring in A, B, C, D, t over Rational Field
sage: L(t=R2.4).parent()
Multivariate Polynomial Ring in A, B, C, D, t over Rational Field