You have said it all. Yesterday was the first time that I have seen Arsenal fight and win an important match with all determination. Look at how Arsenal played Madrid as if they're a mediocre team and they couldn't do anything but to accept the shame and disgrace.
Arsenal had an amazing abnormal match against a strong opponent in Champions League because Arsenal can have big problems with their confidence and mentality that are important to play with and beat strongest opponents. This big win shows how Arsenal can play their best matches and it will increase their confidence before the second leg match.
I don't see any hope for Madrid in the second leg because it's like a mission impossible for them since Arsenal wouldn't just sit down fold their arms and watch Madrid win them by scoring four goals without retaliating back. I can't wait to see Arsenal in the semifinals.
You can say it with other clubs but it can not be correct with Real Madrid. They were able to comeback several times before and they will play a second match with belief that they will be able for a come back after this terrible result in the first leg.