There are several sayings and idioms similar to 四海一家 which reference "四海".
Does the 四海 actually refer to 4 specific oceans such as the Indian, Pacific etc. or is it more of a metaphor for all of the oceans in the world?
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As Fivesheep pointed out, “四海” means "the whole world". The word "四海一家" means "the whole world is one family". It is used to expressed that "we are family. we shall respect each other. we shall unite", something like this, to convey the politeness from the overseas Chinese people to the local people(in your case, the Australian people).
I believe this word comes from the book of 《论语》. It records a speech from a Confucius's disciple "子夏": "四海之内，皆兄弟也。",which means "[if you are a gentleman and you are polite to other people],all the people in the world are your brothers".
四海如家 doesn't sound like an idiom. I think 四海为家 is what you are talking about. In ancient china, people didn't have the idea of oceans named Pacific, Indian... They tended to believe china was surrounded by 4 seas, 东海, 南海, 西海, 北海(if you know the chinese myths you might have heard of 四海龙王). In the idiom, 四海 means anywhere, or the whole world (Ancient chinese also believed the territory of Ancient China was the whole world).