As a practical matter, to enter the fourth jhāna, let go of the pleasure of the third jhāna, and hopefully when you do so, there will be a sense of things starting to physically drop down. Go with this sense of dropping down, and continue to let it drop. In the third jhāna, you may find you have a faint smile, a Buddha-like smile. If so, all you have to do is relax the muscles of your face. Both the smile and the sense of pleasure disappear; then usually a sense of things starting to drop down follows. Just stay focused on that sense of dropping, which can continue for quite some time.
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