Are there any online resources for learning Chinese 4-character idioms
(As BertR pointed out, 独一无二 is a Chengyu; the other two are not.)

Unlike Chengyu, 4-character idioms don't have any significance over 3-character idioms, 5-character idioms or any other idioms, in any means other than the rhythm role it plays in the sentence. They're just valid combinations of words, you can totally replace parts or make your own, and there are millions of them. For example 由此可见 is literally from this (we) can see and we also say 由此可知, 由此可得, etc. Similarly, for 一般来说 we also say 一般来讲, 总的来说, etc. 

If you want to look them up, there are some good websites mentioned in the other answers or Bing/Google translation (I personally find Bing translation more accurate). If you're looking for a list of them, I am almost sure there isn't one, as they can be created ad hoc during a conversation.