It's just that photographers 'focus' on the hexagonal ones

That is the current accepted wisdom.

As for 4 or 6 pointed crystals. That may depend in the thickness of the water, and if you saw enough of the snowflake bubbles you would see fern like structures. I would add-in the effects of ambient and water temperatures, and the concentration of the soap (which may be nucleating the crystals anyway) and talking of nucleation sites - what shape are the nuclei? What are the dissolved salts?
ie Sodium Chloride, Chlorine and Sodium Fluoride crystals look to be predominantly square. Chlorine, and Fluorine are very often in tap water.