Most likely, he isn't rearing because he's confused, he's rearing because he is a stubborn stud with too much energy. He didnt forget how to behave on the lunge line, no matter how long it's been since the last time he was lunged. Don't feel bad or think that you pushed him too hard, he should be accustomed to getting worked. Unless he is genuinely frightened, there is nothing wrong with taking a horse out of his comfort zone. He needs to learn to respect you, no matter what the circumstances. Don't accept his rearing and CERTAINLY do not give up the next time he misbehaves, that will only reinforce the behavior and make him think that it is a good way to get out of work.
It is perfectly acceptable to lunge your horse without a round pen. As long as the footing is sound and there aren't any strange obstacles, he should be fine to be excercised on the lunge.
I would advise that you look up a trainer in your area to get some help with lunging him, considering he is a stud.
If you don't already have a surcingle and some side reins, you should purchase some. You'll also need a bridle, a lunge whip, and a lunge line. You can put the side reins on the lowest notch initially because he is only 4, and then work your way up to a more collected head set as he gets older. The side reins should keep him from rearing because he wont be able to use his head against you. You should take the reins off the bridle so that he doesn't break them.
Again, I suggest you get in contact with a trainer who can help you, and don't allow the stud's unruly behavior control you.