OK first, the bad news: you can put any of these in the sentence. I guess you've already searched the phrase and found native examples of all four. However, a couple of these phrases are fairly rare. The natural collocation (ie. these words simply go together; that's all) that we use is "I've heard it said". The phrase basically means, "I heard it somewhere a few times, and this seems to be the opinion of a few people".

I have heard it said.. is most common