Not sure, tho I guess if you were to fold the piece of A4 slightly so that the validation stamp shows somewhere on the paper, then that would be serve as proof of validation. Others will doubtless know
Just a quick heads-up for anybody going to Rome and planning on getting the Leonardo Express train from the airport into Rome Termini, and also on the return journey: make sure you validate your paper train ticket before getting on the train. The ticket inspectors are extra vigilant and unwary...