Richard Zen:
MCTB
Concentration practices (samatha or samadhi practices) are meditation on a concept, an aggregate of many transient sensations, whereas insight practice is meditation on the many transient sensations just as they are. When doing concentration
practices, one purposefully tries to fix or freeze the mind in a specific state, called an “absorption,” “jhana” or “dyana.” While reality cannot be frozen in this way, the illusion of solidity and stability certainly can be cultivated, and this is concentration practice.
Concentration practices can be fun but there's always some stress chemicals involved just like chasing after any goal. Insight practice gets you to see things how they are and to get disenchanted with them so you are less addicted and rely less on that part of your brain.
To be quite clear, if one is using Jhana as for "fun" and not building it as a positive training to see things as they are, then one is staying in the first three jhanas, likely jhana 2,
or just passing through the natural exhiliration of some of the arupa jhanas--- that exhiliaration passes when they are naturally seen to be unreliable/impermanent sources of happiness, but which experiences do focus one wholesomely on here and now.Jhana is just a very wholesome and welcoming way to train one's brain into a temporary condition of equanimity and from that equanimity one can see what a mind can do, what a life can experience, and become wholesomely detached to many, many stories and just appreciate life as it is right now with reliable peace of mind.
The fourth jhana is not just a launch pad for the arupa jhanas; the condition of suffusive equanimity (fourth jhana) is both a requisite condition for the arupa jhanas to arise (and pass) and it is the conditioned means for seeing clearly (insight, vipassana) the mind itself as well as including such dhammas (phenomena) of the arupa jhanas, but also including its conventional ability to continuously sprout thoughts and feelings with various tones (e.g.,depressive, joyous) and its addictive-and/or-repulsing impulses. Voku, Breath is an excellent object for pleasant concentration. All concentration can start stressfully because one is generally well trained for stressful mentation when starting meditation.
Best wishes in your practices, all.