Hmm, that's an interesting thought too, though I do recall some situations were Kira did serve as first officer on the defiant.
You are right though, clearly she was promoted between Season 3 and Season 4; not season 4 and Season 5 as I thought.
I wonder if she was promoted to be the XO of the Defiant and then Worf came on board and Sisko offered it to him ? Worf clearly makes statements on the station that suggests he's Sisko's XO later in that season.
There's a lot of times throughout the series where it seems like Dax, Kira and Worf are all but stepping on each's toes being the XO without getting in fights over it. It's a little weird --- to say the least.
Beyond that --- am I wrong in thinking Dax might have been the fastest promoted person we know of? I mean comparatively, we had poor LaForge who graduated the same year as Riker starting on the enterprise as an Lt Jg compared to Riker being a Cmdr. We know Riker was fast tracking, but that means it took LaForge 10 years to get to Lt Jg ...
At the start of the series, Dax had only been out of the academy for a short-ish time. Not longer then a year to a year and a half tops.