What is wrong speech according to Buddhism? Is something a lie if I don’t have bad or evil intentions but what I said turns out to be false? I’m trying to protect my five precepts but sometimes when I say something it turns out to be false but I just said what I know at that time. I wasn’t trying to deceive or tell lies. I just said what I know but I found out later that I was wrong. So is that a lie? I didn’t have any bad intentions. I just said what I know and I found out later that it was false.
I don't think the fourth precept is defined in detailed in suttas. Actually I don't think any of the precepts are defined in detail -- perhaps they are in a commentary.

Something like that precept, though, is defined in the vinaya (for monks) -- see Chapter 8, page 298, of The Pāṭimokkha Rules
Translated &amp; Explained
by Ṭhānissaro Bhikkhu, which includes:


  
  Intention: the aim to misrepresent the truth; and
  Effort: the effort to make another individual know whatever one wants to
  communicate based on that aim.
  




But I don't understand your question saying, "I just said what I know and I found out later that it was false" -- because, how could you "know" something that's false?

I think (I hope) I'd tend to phrase that as, "I think that so-and-so", or "Someone once told me so-and-so", or "I guess that so-and-so", or "Such-and-such a reference says so-and-so" -- not, "I know that so-and-so is true" when I'm not sure.

If I'm not sure about something, then conveying more certainty than I have (e.g. by saying, "X is definitely true") is in my opinion misrepresenting the truth about how good my knowledge is, how much certainty I have, how trust-worthy it is.

Anyway you should probably read that chapter of the vinaya, it includes various details which I won't quote here.

The vinaya says it's wrong to break a promise, too. In my opinion (though, I'd guess, not according to Buddhist scripture) that means you should avoid kind of implying (perhaps an implied promise) that something is true when it isn't -- so for example if you're not certain then don't imply you're certain.

Since you mentioned "wrong speech" you might also want to read about "right speech" more generally, i.e. one of the factors of the noble eightfold path (I don't know whether that's exactly the same as the fourth precept):


Right Speech (quotes from Suttas)
Right Speech (a little essay by Thanissaro Bhikkhu)