Hi and welcome to BYC. It is certainly possible to xray a chicken to detect an injury, but a good vet could likely determine if your hen is is injured (and where) through palpation. I wouldn't lay her on her back again, since doing so can put stress on internal organs and may further injure her back if that is the problem. If she can pull her legs inward towards her it's probably not Mareks. The fact that she is a red sexlink (high production layer) makes her very prone to reproductive issues. In addition, if she hasn't eaten or drank for 4 days those are additional strikes against her. I am tagging some of byc's medical experts
@azygous @Wyorp Rock @coach723 @TwoCrows @Eggcessive in hopes its not too late to save your sweet Ethel.