4th density is "free" of the forgetfulness of 3rd between cycles ( this is stated explicitly in transcripts, maybe someone can give direct quote )
I believe it is directly explained in the Ra material (Llresearch's Law of One):
Session 21
February 10, 1981
QUESTIONER: When the 75,000-year cycle began, life expectancy was approximately nine hundred years, on average. What was the process and planning mechanism, so to speak, of reincarnation during that period, and how did the interincarnation period of third-density physicality apply to the growth of the mind/body/spirit complex?
RA: I am Ra. This is a very complex question. Let's begin the explanation.
The incarnation pattern of the mind/body/spirit complex that initiates third-density begins in darkness, for you may consider your density one of sleep and forgetting, as you might say. This is the only plane of forgetting. The third-density entity must forget so that the mechanisms of confusion, or free will, can operate upon the newly individualized consciousness complex.
Thus, the beginning entity, in complete innocence, is oriented toward animal behavior, using other beings only as extensions of the self for the preservation of the entire being. Slowly, the entity becomes aware of its needs, so to speak, which are not those of the animal; that is, which are not necessary for survival. These needs include: the need for companionship, the need for laughter, the need for beauty, the need to know the universe. These are the needs of the beginning.
As incarnations begin to accumulate, other needs are discovered: the need to negotiate, the need to love, the need to be loved, the need to sublimate animal behaviors to a more universal perspective.
During the first part of the third-density cycles, incarnations occur automatically and occur rapidly after the cessation of the energy complex from the physical vehicle. There is little need to review or heal the experiences of the incarnation. As the energy centers begin to activate to a greater degree, the experience during incarnation that relates to the lessons of love increases.
Therefore, the time between incarnations is lengthened, as you might understand, to give appropriate attention to reviewing and healing the experiences of the previous incarnation. At a certain point in third density, the green ray energy center is activated, and at that moment, incarnation ceases to be automatic.