Your poppet looks OK to me, but without seeing it first-hand, it's only a guess.
Make sure you are using a fresh O-ring on both the dip/gas tubes, and another fresh one on top of the post. Tighten the post reasonably tight but do not overtighten.
"Thinning" at 4 days? I assume that it's near the end of fermentation and not carbonated. Before determining if it's thin or not, you need to carbonate it at the desired volume.
If it's thin after carbonation, you need to review your mash temperature and increase the temp to increase body.
in THIS case Topo Chico is sparkling water. But Coca-Cola bought them a while back and is selling hard seltzer under that name now. The fact that the bottle says 0 calories leads me to believe it's sparkling water and not hard seltzer.