I would say it's just a little bit of "cold feet" that made her say what she had said. She's had time to think about it and said that she did loves you!
Believe in relationship, troublesome times come and go... after 4 years I believe this will pass by too!
Moving in together is a big step in any relationship, it's only natural for someone to get cold feet and as a result question themselves (their feelings), this she did and after time, found her answer!
I believe that after 4 years of being together you're both strong enough as a couple to pull through this time apart, the plan of moving in together is testiment to this!
I don't think that a 4 year relationship is weak enough to buckle at a hurdle of this height, I believe it's something you'll both jump over together!
Best of luck!
EDIT - Regarding her meeting her friend, I can honestly see it being nothing at all! Because of the move she had questioned herself in being ready for this commitment. She met her friend and spoke to him about it. She's come back after being with him saying that she loves you and wants to give it a go!
I reckon he's been a friend and spoke to her about it, the flirting is something as innocent in trying to make her feel better about herself and to cheer her up!
I don't think she would have come back saying she wanted to give it a go otherwise.
Now you just have to let her have time! She may be questioning her thoughts on your commitment after you spoke about reading her txts... give her time, 4 years counts for more than people know!
I have faith that it'll be alright, just give it time and then talk!