You think Ruoxi fell in love with 4th illogically? Really? I'm so surprised. :O I thought it was very evident and very logical why she fell in love with 4th so much more than 8th. Why did you think it was illogical?
In regards to the wife thing. Perhaps I made too much emphasis on Minghui being a lioness, but my actual point was, it's not because Ruoxi was afraid of Minghui or the prospect of being one of many wives, but through that event, she realizes 8th will not protect her or put her first. Remember you said Ruoxi did not understand why she couldn't face death with 8th? Shouldn't you be able to if you really loved someone? Well, I think ti's because the Minghui thing had put doubt in Ruoxi's mind (subconsciously, because I don't think she realizes it.) if she was as important to 8th as she thinks. As a modern woman, I think Ruoxi has that idea that if you love someone, then you'd place them first and protect them with all your abilities. But 8th clearly can't do that.
8th's brushing off of Minghui's bullying towards Ruoxi's sister shows some insincerity what 8th is making his love to be like. Ruoxi knows, that he will do the same if the same thing (the bullying) happened to her; there's no way he'll come to her defense and cross Minghui. For him it's such a minor incident. As Ruoxi explains to 13th, why she didn't fall in love with him, she says that she tends to love those who take that first step to show affection, and who gives a lot to her (love wise). I guess you can interpret that as, the more one loves her, the more she'll love back. Sincerity is a big factor here. Therefore, she didn't love 8th as much because his love wasn't deep enough or sincere enough.
Yes, I think 4th was very surprised to find that Ruoxi loves him strongly back as well, because up until then, he most likely felt that she was with him primarily for protection rather than out of pure love. Which makes me like him more because he promised to be that tree for her in even when he knows she is primarily using him for protection. Although he promises it in such a bossy way. lol: 他微微笑了下说:“你既已戴了我的簪子,又说了要嫁我,以后就莫要再想别人了!”(He smiled slightly and said, "Since you put on my hairpin, and said you want to marry me, from now on you can stop thinking of anyone else!"). I think it really shows how much his feelings are for her. So Ruoxi definitely has not overestimated her own worth; she just thinks she did.