日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

Would somebody kindly explain the significance if 一　ｖｓ　一つ？

Likewise, could somebody explain when to use any pronunciation over another?  I know there are exclusive instances where kun pronunciations are used in compounds, and I know chinese pronunciations are usually used in compounds, etc., but the idea behind using 四　over 四つ　eludes me.