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This works perfectly:

sage: var('a,b,c,d')
(a, b, c, d)
sage: t = (a**2+b**2)*(c**2+d**2)
sage: t
(a^2 + b^2)*(c^2 + d^2)
sage: expand(t)
a^2*c^2 + b^2*c^2 + a^2*d^2 + b^2*d^2

Maybe you made a mistake in the parentheses ?