High pair or 4 to a flush?
Keep the high pair. Exact EVs on 9/6 Jacks or Better: a pair of jacks is worth 1.5365 per coin vs 1.2553 for the flush draw — guaranteed money beats the 19% draw.
The exact numbers — J♣ J♦ 4♦ 8♦ Q♦
EVs are exact — every possible draw enumerated, not simulated — at max bet on 9/6 Jacks or Better. Check it yourself: the analyzer runs the same solver in your browser.
See all 32 holds of this hand ranked →Why
The high pair already pays even money before the draw starts — you're freerolling on trips, two pair, and quads. Breaking a made even-money hand for a 9-out draw is paying full price for something you already own a piece of. (The one exception in the whole game: 4 to a ROYAL outranks the pair.)
Chart line #6 of 18 (high pair) vs #8 (4 to a flush). See the full solver-optimized strategy chart, or drill hands with exact-EV feedback until plays like this are automatic.