(My feelings about Angel's actions in this episode are well known here on the Boards, as well as elsewhere, so I doubt that my comments here are going to come as a shock to anyone.)
I think that Buffy's initial reaction would have been shock, anger, and an acute sense of betrayal. Possibly with enough time to cool down and consider the situation more thoroughly and, maybe, more dispassionately, she might have come to see and accept some of Angel's logic in making the decision that he did, but still, especially if she had learned about it in Season 6, after she had sacrificed herself in order to save, not Angel, as the Powers had told him, but Dawn, there would have been a sense of futility and a feeling of being forced into an inescapable fate (remember, I said that this would depend on her finding out in Season 6, when her depression was at its peak) which would have only served to strengthen her already near suicidal mood.
Probably, eventually, there would be some sort of reconciliation between Buffy and Angel, but her sense of total trust in him would have been lost, for a very long time, if not forever. She would still love him. Buffy will always love Angel. But he would no longer be her White Knight.
As to whether or not her relationship with Riley would still have taken place if she had learned in Season 4 instead of later, yes, almost certainly it would have. Remember how she described him in "Five By Five?" "Someone who can I love. SOMEONE WHO I CAN TRUST." I don't think that it was either an accident or simple coincidence that she chose those words. Trust is a major factor for Buffy, in all of her relationships, whether with friends or lovers. At least initially, if not forever afterward, Buffy would never get over a feeling of betrayal for what Angel had done.