In fairness, the Episode IV: A New Hope bit was added for the 1981 re-release.
Yes this is true.
In fact, George Lucas originally wrote a movie storyline that was intended to be just one movie (Which was released in 1977). However, once he began writing the screenplay for the movie, it got to be too long, so he decided to break it into multiple films (3, representing the original 3 Star Wars movies). He completed the screenplay for just the first, and completed the film. However, in the meantime he had to develop a story line to explain the history of the characters in the movie. It wasn't really his intent to make movies out of the backstory, but it provided a history of the fictional worlds that he had created to explain how the circumstances in the story came to be. After the success of the first film, Lucas considered converting his backstory into more films. So after a short period of four years, he established the original Star Wars as the fourth chapter in 1981 and eventually produced the three prequel films many years later.
So was it Lucas's original intent from the beginning to create a prequel series? No.
However, was their enough pre-established back story information to do a prequel series? Yes.
Did it seem natural within the story that there might have been some prequel story or story lines to tell? Yes.
Was the re-wording of the 1st Star Wars (in the opening crawl) as the 4th chapter established early enough (as if it was supposed to be there)? Yes. I believe four years is a short enough time to establish new rules for a fictional universe.
Although Lucas didn't intend to make the prequels from the very beginning. Things flowed together naturally enough (in the early years of Star Wars) as if the prequels were supposed to be a part of the saga, though.
I mean, if he decided putting chapter numbers into the original trilogy after it was totally completed... then it would have been a bit after the fact. However, seeing he established the numbering of the chapters close enough to Empire Strikes Back's release, it helped redefine or fine tune the original story early enough at the beginning of the Star Wars saga (as if it was originally supposed to be there).
I mean, the numbering of Star Wars was kind of like a work in progress, for me. At least that's my take on it, anyways.
In other words, it was by a lucky coincidence that he had the stones already set into place for a prequel series (even though it was not originally conceived of at the very beginning).
I mean, does it really matter if he thought of making a prequel series a few years later or at the very beginning?
For me, there is no difference and the result is still the same. Whether Lucas intended to create a prequel series or not; the stones were already laid out and it was quickly established for us to expect a possible prequel series some time in the future if things went well.