Numbers in Japanese... can someone help me out?

The readings of "shi" for 4 and "shichi" for 7 derived from Chinese. Such readings are called "on-yomi" (sound-reading), because they are read in accordance to the sound of the Chinese language. They might be different from today's Mandarin, but it is said that such reading existed in the Central China in about 3 D.C..

"Yon" for 4 and "nana" for 7 derived from essential Japanese, which has existed before Japan introduced the Chinese culture. Such readings are called "kun-yomi" (orthodox-reading). When the kanji was imported from China, we allocated this Chinese character to the corresponding Japanese word and read it in correspondence to the Japanese sound. That is the origin of kun-yomi.

As you have mentioned, "shi" and "shichi" are the main readings. While "yon" or "nana" is used when the rhythm in reading the sentence needs such reading, or when accurate communication without mistaken listening is needed.

We hear such readings in stations and so on. For example:

●Nana ji yon-juu-nana fun hatsu Nagoya iki wa yon-ban sen desu.
(The 7:47 train bound for Nagoya is on track No.4.)

There are two sets when you count to ten.Native Japanese,and those borrowed from the Chinese long ago.Japanese is:
1 hitosu
2 futatsu
3 mittsu
4 yotsu
5 itsutsu
6 muttsu
7 nanatsu
8 yattsu
9 kokonotsu
10 to
Four is usually said as 'shi'.And 7 as shichi.
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If you are just counting, then it's okay to use either one. Japanese people will often use yon instead of shi in this case because shi is also the word for death in Japanese. You'll actually notice that there are few sports players with the number 4, and even some hotels without a fourth floor for this reason. Maybe nana is slightly more common than shichi, if only because it's pronunciation is harder to mix up with shi. I suggest using nana and yon, because they sound less like other numbers, so it will be easier for other people to understand your pronunciation.

There are certain instances where one or the other can only be used, to match with counter particles and such. For example, April can only be called shigatsu, and July can only be called shichigatsu. You should just memorize those or use ones that you have heard native speakers say.

As for why, it comes from the mixing of Chinese and Japanese, as far as I know. There were at one time more names for all the numbers, like hitotsu, futatsu and so on.

The fact there are two different numbers also confused me. I thought it was because of the dialect. If I find the answer I'll be sure to tell you. I've always learnt it as shi and yon, though.

if you're saying the numbers individually, it's ok, but once you start saying numbers in a string, ie. phone number, shi and shichi can get mixed up. You'll actually see this in the Chinese language, too, with 1 (yi) and 7 (qi).

I know when you learn Japanese they teach you shi and shichi, but actually I usually hear yon and nana used. I'm not sure of a rule or anything, but in most cases you're gonna use yon and nana.

According to Lonely Planet the recommended way of saying 4 is yon and 7 is nana and 9 is kyu (as opposed to ku). The numbers are from Chinese. But as for why there are two different versions? Dunno soz.

Ughhh...I know the feeling! Sorry i cant help you.