Why is 相 pronounced with a fourth tone in some cases?
相

[Xiang1]
surname Xiang

[xiang1]
each other,
one another,
mutually

[xiang4]
appearance,
portrait,
picture

You should look into getting a Chinese reader. http://www.loqu8.com has one for free that is really good. It uses the CC-CEDICT dictionary which is really good. If you would like more concrete examples, they're all going to be in Chinese, but there they are nontheless. http://stroke-order.learningweb.moe.edu.tw/advExplain1.do?big5=ACDB

Enjoy.^^