Solving for inverses with restricted domains

Hi

I was trying to find the inverse of a function where the domain has been restricted. For example

solve(x=subs(x=y, x^2+8*x), y, useassumptions) assuming x::RealRange(-4, Open(infinity));

and my hope was that including the assumption would result in a single solution but it still gives me two solutions as it would without the assumption.

I would like to algorithmically end up with a single solution. Currently I am only working with functions that have variables to the power of 0, 1 or 2 and additionally square roots, but a more general method would be optimal. What am I doing wrong?