Hi anthony,
But I am having no luck trying to get out of the 4th jhana into 5th, boundless space. I am trying to place attention on space, or launch or catapult upwards, but not even sure what space is. Sometimes when I think I have been getting there, there are some strange warping or fear sensations... perhaps I have some sense that I'm almost afraid to leave the body and breath behind...?
Would maybe having a different object of meditation help? eg visualising light? not ever made a concerted effort at visualising lights...
After the 3rd concentrated state, (third jhana, sukha) I think one's
provision direct action (meditative determinationism (e.g., "I will get to 2nd jhana"...)) must be released
into just sitting (this is deliberate, applied equanimity which IMO cannot be forced, but is grown over the practice up to this point); it is now intention
prior to/surrounding one's practice and
still wholesome conduct that enables the mind to enter 4th jhana and thereafter arupa jhanas on its own, naturally. A practitioner is, as they say, just setting up the prior conditions for that jhana and the arupa jhanas to happen. (Wholesome conduct has many benefits, but one of them is one can sit and not be dealing instead with remorse or other troubles that one brings to oneself.)
Fear around the body is very natural. In fact, breathing here can get very, very reduced and maybe the medulla oblongata in the lower brain stem is detecting lower o2 and co2 in the arterial blood: the natural body response here in a healthy person is for a sympathetic nervous response to "sympathize" with low, vital nutrition (02) and to make you "nervous" and increase breathing rate. So anxiety may be coming up naturally and with practice, you will not prioritize that occurrence and the mind will continue to sink. (NOTE: I like anapanasati for the full jhana study and it worked for Gotama...) [1]
So the fear commonly reported here may be a natural response to respiratory reduction and the mind getting used to it, to not panicking (and an indicator that your mind is approaching suffusive equanimity, where the driver (you/me/the practitioner) is very much dormant, not directing any thought or creating "deterministic meditation", which would be wanting and trying to get 4th jhana and the consequent arupa jhanas).
That's a theory I have

I would add: mentally, I have lurched out of this practice area unable to move or re-start breathing. The body was very, very calm so there was the ability to see the body kicking in its physical response to non-breath, the me-mind rising-up suddenly saying "Why can't I move and breath?", and then there was neutral awareness about all of it. "I" quickly subsided again calmly (hey, if you can't move or breath there's not much to do) and the mind re-entered equanimity and then emergence from the state was gradual and calm.
Just a reminder: insights in 4th jhana and the arupas can be particularly useful, but it's important to know also that these are just dhammas releasing dhammas (a created object (insight) is arising to release a prior created object (let's say a deluded behavior)). It's really easy to get attached to insight from the arupas...
Best wishes
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[1] I am not dismissing fear/anxiety here as a mere physiological phenomena. It is key to pay attention to the fears here. It is by closely attending the emotional arisings that the mind eventually calms, like a muscle relaxing naturally. After pracitice one may spend hours investigating how one is living, one's goals, one's choices. So this is a stage one would not want to try to blitz through by just thinking, "It's only emotion" because if one ignores an aversive residue of mind here (or a craving one, too), there's lateral movement, not depth in the practice, and who wants siddhis coming up in a fearful mind? So this anxious area is very useful as much of daily life involves acting out fear/anxious concern in regards to one's own survival/death/status/well-being...