The jhanas have both 4 and 5 factors actually. In the original suttas they were described as having 4, but in the Abhidamma as 5. Why is this? Leigh Brasington hypothesizes in an interview in the book Samadhi with Richard Shankman.
I'll copy the relevant statements under fair use.
"The definition of what constitutes a jhana has, in a thousand-year period, progressed to a much deeper state. We might ask how this happened. Think about who was preserving the Buddha's teaching during these thousand years. It's a bunch of guys hanging out in the woods--no TV, no women. They've got just their minds to work with. And so they start working on the jhanas. And if somebody can take it a little bit deeper, obviously he's doing it "better." The natural human tendency is, "Well, if I can do it better than you are doing it, I'm doing it the right way, and I'll teach you to do it my way."
"So I would guess that over time jhana evolved from pretty serious states of concentration to the extreme states that we find preserved in the Visuddhimagga. The Abhidhamma seems to be somewhere in between, but obviously getting very, very deep during that period, since people were able to see their mind-moments and so forth." p. 159
My own thoughts here:
It very much seems that one can cultivate the jhana factors at different levels of concentration, and the absolute experience of the jhana state(first and even beyond) will depend on the level of concentration cultivated before. Leigh states(also in his Samadhi interview) that the Jhanas he teaches are probably accompanied with even less concentration that that of in the suttas, and that it would be unlikely to reach sutta level Jhana with less than a month retreat. With sutta quality jhana, one is still able experience the three characteristics and come to ultimate truths, whereas in Visuddhimagga jhana, one must leave the state and then investigate reality.
Leigh's jhanas are perhaps best contrasted to B. Alan Wallace's, who spent some long time as a Tibetan monk under the Dalai Lama, and recently wrote "The Attention Revolution." Here, he states that the level of concentration to develop Jhana would be at least 3 months, but, more likely, years. In fact, he assisted in a year long samatha retreat in the late 80's and he reported that at the end, none of the students had reached jhana, but they were able to sit in one pointedness for many hours at a time. Certainly this is highly related to the jhanas in the Visuddhimagga perspective.